In rodents and non-human primates, there are substantial reciprocal interconnections between the PFC and the MD (Krettek and Price, 1977; Goldman-Rakic and Porrino, 1985; Groenewegen, 1988; Ray and Price, 1993; McFarland and Haber, 2002; Xiao et al., 2009). Higher order thalamic structures, like the MD (and the pulvinar) receive inputs from different cortical layers. The majority of projection neurons to the MD originate from layer VI and V (Giguere and Goldman-Rakic, 1988; Yeterian and Pandya, 1994; Xiao et al., 2009); mainly from within the deep regions of these layers. The cortical layer V pyramidal neurons also have branches of long descending axons going to motor centers (Guillery, 1995; Guillery and Sherman, 2002). Guillery also proposed that these higher order thalamic nuclei play a key role in cortico-cortical communication and higher cortical functioning (Guillery, 1995). Thalamic neurons innervated by cortical layer VI project focally to the middle cortical layers and thalamic neurons innervated by cortical layer V project widely to the superficial cortical layers which are involved in cortico-cortical communications (Jones, 1985; Xiao et al., 2009). In addition, there are nonreciprocal components to the thalamo-cortical links, indicating a dual role for the MD in integrating basal ganglia outputs within specific cortical circuits (see below) (McFarland and Haber, 2002; Haber and Calzavara, 2009), as well as mediating information flow between cortico-cortical structures via this transthalamic route (Guillery and Sherman, 2002; Sherman, 2005, 2007; Sherman and Guillery, 2005). Glutamate is the main neurotransmitter of communication between thalamus and cortex (Sherman, 2013).

The major outputs of the MD are to the medial and lateral prefrontal and orbital frontal (OFC) cortices, and in some neuroanatomical tracing studies in rats, the medial PFC is said to be defined by the projections received from the MD nucleus (Groenewegen, 1988; Negyessy et al., 1998). Thus, these interconnections between the MD and PFC are segregated based on the subdivisions within the MD (see Figures 3A–C). The MDmc-PFC projections are almost exclusively reciprocal between the MDmc and the OFC and ventromedial PFC (vmPFC: areas 14, 25, 11, 13, and 12) but there is also a nonreciprocal input from ventrolateral PFC (VLPFC: area 45) and medial PFC (dACC: area 32 from the ventral and caudal aspects) (Preuss and Goldman-Rakic, 1987; Russchen et al., 1987; Barbas et al., 1991; Bachevalier et al., 1997; McFarland and Haber, 2002). Some of the midline nuclei [e.g., the intermediodorsal (IMD) and the paraventricular (PV) nucleus in rodents, see Figure 1] are also reciprocally connected to the OFC (Groenewegen, 1988). Thus, the MDmc and these midline nuclei have been regarded as a neuroanatomically functioning unit in rodents (Mitchell and Dalrymple-Alford, 2005). The MDpc has reciprocal connections with the dorsolateral PFC (DLPFC; areas 9 and 46) and area 10. There is also non-reciprocal inputs to MDpc from OFC (area 12, 13), VLPFC and the dACC (supracallosal area 24 and from the dorsal and rostral aspects of precallosal area 32 and 14) (Preuss and Goldman-Rakic, 1987; Russchen et al., 1987; Barbas et al., 1991; Bachevalier et al., 1997; Haber and McFarland, 2001; Erickson and Lewis, 2004). The most lateral parts of the MD that are combined with the ILn diffusely project to the PFC and dACC (supracallosal area 24) and exclusively to the frontal eye fields (FEF); the most prominent projection however is the topographically organized input to the basal ganglia (Preuss and Goldman-Rakic, 1987; Barbas et al., 1991; Bachevalier et al., 1997; Erickson and Lewis, 2004; Erickson et al., 2004).

In non-human primates there are connections from the association cortex of the temporal lobes [i.e., the entorhinal (ERh) and perirhinal (PRh) cortices] to the MDmc (see Figure 3A) and midline thalamic nuclei (Aggleton et al., 1986; Russchen et al., 1987; Saunders et al., 2005). These mainly course through the ventroamygdalofugal pathway and the inferior thalamic peduncle. In the rodent brain, it appears that only the rostral portion of area 35 of the perirhinal cortex projects to the MD (Burwell et al., 1995).

There are also amygdala (Amyg) projections to the MD (see Figure 3A). The central nucleus and basolateral nuclei project densely to the MDmc in monkeys and medial segment of MD in non-primates (Aggleton and Mishkin, 1984; Groenewegen et al., 1990; Krettek and Price, 1977). These projections from the amygdala to the medial MD are much sparser than the amygdala projections to the striatum and PFC (Jones, 1985). Krettek and Price (1977) reported that the fibers from the caudal part of amygdala terminate rostrally in medial MD and those from the rostral part terminate more caudally and ventrally in the medial MD. In turn, it has been documented in rodents that medial parts of MD project back to the basal grouping and anterior cortical nuclei of the amygdala, while the midline nuclei project to the central nucleus and the rostral part of the basolateral nucleus of the amygdala (Groenewegen et al., 1990).

In contrast, the central MDpc and more lateral parts of the MD (see Figures 3B,C) do not directly interact with the MTL and amygdala.

Monkey studies have demonstrated that aspiration lesions to the MD (i.e., typically including the magnocellular and the parvocellular subdivisions and other medial thalamic structures as well as potential fibers of passage passing through this region) cause impairments in recognition memory, deficits in new learning of object-in-place (OIP) discriminations and object-reward associations. These lesions also produce impaired performance in the spatial delayed alternation task and delayed response task but not in object reversal (associative memory task) and visual pattern discrimination (Isseroff et al., 1982; Aggleton and Mishkin, 1983a,b; Zola-Morgan and Squire, 1985; Gaffan and Murray, 1990; Gaffan and Watkins, 1991; Parker et al., 1997; Gaffan and Parker, 2000). Other studies have also highlighted how interactions between interconnected structures of the amygdala, vmPFC and MD are important for postoperative new learning of two-choice visual discriminations associated with differing amounts of food reward (Gaffan et al., 1993). Despite this extensive range of deficits linked to damage in the MD, it did not appear that MD lesions by themselves were the critical source of dense amnesia linked to cases of thalamic and diencephalic amnesia suffered in patients. For example, Parker et al. (1997) found that bilateral ablations to MDmc did not produce recognition memory deficits as severe as those reported after bilateral perirhinal cortex ablations, and the animals were also not as markedly impaired as amnesic patients with recognition memory deficits (Aggleton and Shaw, 1996).

Parker and Gaffan (1998) proposed that ablation of the MDmc in primates produces hypoactivity in the PFC and, therefore, the deficits in cognitive testing after MD lesions might be ascribed to frontal dysfunction. Given the extent of dense reciprocal connections between the MD and PFC, it makes sense to propose that damage to the MD may result in dysfunction within the PFC and that this disruption causes deficits in cognition and memory (Isseroff et al., 1982). The PFC is associated with higher order cognitive functioning, often labeled “executive functioning” in humans. It has been suggested that lesions to the MD could disrupt pathways leading to the PFC and may affect processes that are typically governed by the PFC, including attention, inhibition, planning, coordination, and strategy selection, which could then produce memory impairments on tasks (Gaffan and Parker, 2000). Certainly all of the above tasks that produce deficits after MD lesions are also sensitive to damage in the PFC (Fuster, 2008; Chudasama, 2011).

More recently, selective neurotoxic lesions to the MDmc have confirmed the importance of this medial subdivision in new learning of OIP discriminations and in a reward satiety devaluation task, as neurotoxic lesions of the MDmc produce impaired performance on these tasks (Mitchell et al., 2007a,b, 2008; Izquierdo and Murray, 2010). However, the same selective lesions to MDmc do not impair the retention of pre-operatively acquired information (Mitchell et al., 2007a; Mitchell and Gaffan, 2008). One such task that assesses retention of pre-operative information is the strategy implementation task (Gaffan et al., 2002). In this task, animals learn a specific strategy for responding to objects presented on a touchscreen in order to receive reward. Pre-operative performance for individual animals is compared with post-operative performance. Animals with crossed unilateral lesions that disconnect the whole of PFC in one hemisphere from inferotemporal cortex in the contralateral hemisphere (PFC × IT) cause impairments on this task (Gaffan et al., 2002) as do bilateral ablations to the VLPFC (Baxter et al., 2009). However, damage to the MDmc did not disrupt the animals' ability to implement the strategy that they had acquired pre-operatively (Mitchell et al., 2007a), despite the strong reciprocal interconnections with PFC and input from IT.

In the same study, another task, the OIP discrimination learning task, was also learnt pre-operatively although in this task animals learn 20 new pairs of OIP discriminations (see Figure 4 for example stimuli of the OIP discriminations) during each session across eight concurrent repetitions of the set of 20 pairs of OIP discriminations. To assess impairments in this task, the performance of each animal is compared during a pre-operative test of 10 sessions and compared, after recovery from neurosurgery, to a post-operative test of 10 sessions. The neurotoxic MDmc lesions caused animals to make more errors during postoperative testing, so it could be concluded that deficits were linked to new learning of information as opposed to the retention of specific information acquired pre-operatively (Mitchell et al., 2007a). Critically though, this evidence demonstrated that the damage to the MDmc was not simply causing widespread PFC dysfunction to account for the observed cognitive deficits. Furthermore, the extent of the deficits on the OIP task were similar across two different lesion studies provided convincing evidence that the neurotoxin lesion technique used by Mitchell et al. (2007a) worked as effectively as the previously used ablation method (Gaffan and Parker, 2000).

In addition, the types of errors made in learning the OIP discriminations produced after bilateral MDmc lesions are not suggestive of problems with perseverative responding during learning (Mitchell et al., 2007a). However, bilateral ablations to the VLPFC do produce perseverative responding during new learning of OIP discriminations (Baxter et al., 2008). This evidence further confirms that damage to the MDmc does not simply produce a generalized impairment in memory by causing dysfunction of prefrontal functioning as had been previously proposed.

However, other studies (e.g., Parker et al., 1997) have shown that damage to the MD does not impair new learning or retention on recognition memory tasks when the stimulus set size is small. So it could also be argued that as the stimulus set size (four pairs of objects) is small in the strategy implementation task, no further learning is occurring. This theory was further tested (Mitchell and Gaffan, 2008) by comparing retrograde amnesia and anterograde amnesia within the same animals using the same types of OIP discrimination stimuli for both types of memory (see Figure 4) from a large sample size of 400 pairs of discriminations. In addition, a one-trial retention test was used to assess memory retention; this test is a pure measure of postoperative retention, uncontaminated by post-operative re-learning (Dean and Weiskrantz, 1974). Interestingly, damage to the MDmc using neurotoxins caused no impairment in the one-trial postoperative retention test. That is, the monkeys with bilateral MDmc neurotoxic lesions showed good retention (i.e., no retrograde amnesia) of the 300 pairs of OIP discriminations that they had acquired pre-operatively, when the errors made during their pre-operative retention test were compared with errors made during their post-operative retention test (Mitchell and Gaffan, 2008). In contrast, the same animals were markedly impaired in new postoperative learning (anterograde amnesia) of a further set of 100 novel pairs of OIP discriminations presented concurrently across sessions (Mitchell and Gaffan, 2008). It was concluded from this evidence that the MDmc is critical for the processing of new information more so than in the retention of information acquired prior to damage.

Further research from our laboratory has extended our understanding about some of the brain regions involved in retrograde amnesia and anterograde amnesia using this OIP retention and new learning task. The effects of lesions to different subcortical and cortical structures have been assessed (Mitchell et al., 2008; Mitchell and Buckley, submitted). One such study assessed the effects of combining neurotoxic MDmc lesions with bilateral fornix transection [a lesion that produces more widespread brain damage to medial diencephalic structures as well as disrupting interconnections with the PFC and medial temporal lobes (Mitchell et al., 2008)]. Interestingly these combined lesions produced dense amnesia for new learning as well as retention, yet still they confirmed that subcortical damage produces more severe deficits in anterograde amnesia than in retrograde amnesia.

Clearly animals with bilateral MDmc lesions have provided greater understanding of the critical role that the MDmc plays in differing forms of memory processing. Animals with MDmc lesions have also been assessed on tasks investigating other cognitive processes e.g., reward (satiety) devaluation. For example, animals with selective bilateral neurotoxic lesions to the MDmc are also impaired on a computerized version of a classic food satiety devaluation task (Malkova et al., 1997) demonstrating the importance of the MDmc within the neural circuit crucial for reward devaluation (Mitchell et al., 2007b) that also includes the OFC and amygdala (Malkova et al., 1997; Baxter et al., 2000; Izquierdo and Murray, 2010). Interestingly though, lesions to MDmc did not impair the animals' ability to learn the 60 pairs of object-reward associations presented concurrently over successive sessions during initial postoperative acquisition training before performing the reward satiety devaluation testing in this task. In this reward devaluation paradigm, the monkeys must first learn postoperatively to link one of the two pairs of objects in each presentation with a peanut or chocolate candy reward, 50% of one of the stimuli from the pairs of objects was rewarded for a correct choice with a peanut (while the other object was not associated with any food) and the other 50% of the pairs of objects with a chocolate candy. Presumably this lack of deficit in new learning during the initial acquisition was linked to the smaller set size of stimuli (Parker et al., 1997) as well as to the salience of the rewards [e.g., bilateral lesions to the OFC and the amygdala also do not impair the initial postoperative acquisition of the object-reward associations in this task (Malkova et al., 1997; Baxter et al., 2000)]. Other studies though have shown that bilateral MD lesions do impair the ability of the animals to associate pairs of stimuli with differing amounts of food rewards (Gaffan and Watkins, 1991).

In contrast, MD lesions do produce deficits in new learning of larger sized samples of concurrent object-reward association problems over sessions, although the lesion does not impair the retention of pre-operatively acquired object-reward associations (Gaffan and Parker, 2000). Damage to parts of the PFC (e.g., crossed unilateral disconnection of PFC × IT lesions) does not impair concurrent learning of visual object discriminations, where the animal learns to associate visually presented objects with food (or no) reward and the presentation of each object pair is separated in time by presentation of other pairs of objects (Gaffan et al., 2002). Interestingly, this same PFC × IT disconnection lesion does, however, severely impair serial learning of visual discriminations, where the animal learns to associate single pairs of objects with food (or no) reward and the presentation of each object pair occurs immediately after one another (Browning et al., 2007). Concurrent object-reward association learning is qualitatively different to learning serial presentations of visual discriminations (Murray and Gaffan, 2006). It remains to be fully tested whether bilateral MD lesions produce dissociable deficits in learning concurrent vs. serial visual discriminations, although as previously shown MD lesions produce deficits in within-session new learning of OIP discriminations (Gaffan and Parker, 2000; Mitchell et al., 2007a).

In rats, many studies assess the rats' ability to forage for food using T-mazes, water mazes and radial arm mazes, taking advantage of their natural curiosity to explore novel environments for food. Many strategies can be used by the animals to complete these tasks successfully (Dudchenko, 2001). One strategy involves spatial navigation based on the use of extra maze cues (e.g., door, windows, lights, posters, experimenter, etc.) within the testing environment (spatial cues) to help guide their optimal exploration and ensure they do not return to the same location twice. Animals with selective neurotoxic lesions to the MD show comparable performance to control animals when they use spatial cues to guide their navigation in radial arm maze tasks (Beracochea et al., 1989; Hunt and Aggleton, 1998a; Mitchell and Dalrymple-Alford, 2005), unless they incorporate a delay (Harrison and Mair, 1996; Floresco et al., 1999) or produce more widespread damage that also includes the AT (Stokes and Best, 1988, 1990a,b; Hunt and Aggleton, 1991, 1998a). It has been proposed that MD deficits in delay tasks are presumably a consequence of widespread disruption to PFC functioning (Hunt and Aggleton, 1998b). Floresco et al. (1999) contrasted working memory performance using a spatial delayed responding task and non-delayed spatial tasks to show that the interaction between the PFC and the MD mediates “context-dependent retrieval and manipulation of recently acquired information.” Furthermore, this study provided evidence, via lidocaine infusions into the MD, to show that MD alone is not sufficient to affect episodic-like memory processing on spatial memory tasks (Floresco et al., 1999). Instead, it is widely accepted that spatial memory processing deficits related to the medial thalamus are governed by the anterior thalamic nuclei and their interconnections to the extended hippocampal system, also known as the Delay-Bryon neural circuit (Aggleton and Brown, 1999).

Other researchers have observed in rats with bilateral MD lesions certain behavioral deficits that could result in memory impairments, for example, an inability to adopt different strategies, or changes in activity and exploration levels or deficits in withholding spatial responses (Hunt and Aggleton, 1998a,b; Floresco et al., 1999; Block et al., 2007; Ostlund and Balleine, 2008). All of these types of deficits are also observed in rats with damage to regions of the PFC (Chudasama, 2011).

Rodent studies have been instrumental in demonstrating the distinct, interdependent involvement of adjacent medial thalamic structures in memory and other cognitive deficits. Dissociable deficits between the MD and adjacent anterior thalamus (AT) have been reported (Chudasama and Muir, 2001; Chudasama et al., 2001; Corbit et al., 2003; Mitchell and Dalrymple-Alford, 2005, 2006). Corbit et al. (2003) assessed the effects of highly selective MD and AT lesions in rats on instrumental conditioning. Rats with either MD or AT lesions were both able to acquire the instrumental performance but during the degradation of the action-outcome contingency test, the rats with MD lesion were unable to demonstrate reliable devaluation effects. This deficit shown by the rats with MD lesions was distinct from the rats with AT lesions and controls, which did not differ, and suggests that the MD contributes to deficits in encoding and/or utilizing the action-outcome association (Corbit et al., 2003). Mitchell and Dalrymple-Alford (2005) have also demonstrated dissociable impairments in rats with lesions of the medial MD compared to the lateral MD or to the AT on various cognitive tests. The damage to the medial MD impairs go/no-go reward value discriminations and recency memory with a 2-h delay, but had no impact on spatial memory processing using an 8-arm radial maze or spontaneous object recognition (SOR) memory (see below). Lateral MD lesions produced mild deficits in 8-arm radial maze performance and recency memory but had no impact on go/no-go reward value discriminations or SOR memory (see below). In contrast, AT lesions produced deficits on 8-arm radial maze performance but they had no effect on recency memory using a 2-h delay. Interestingly, AT lesions do impair the ability to remember the pseudorandom order of six odors (Wolff et al., 2006). This deficit may be linked to the nature of the associative memory processing involving reward that is required in this particular task but that is not present in the spontaneous exploration paradigm used in the recency memory task (see below). Further dissociations in performance have been reported in rats with lesions to the lateral MD or to the AT, with only damage to the lateral MD impairing the rats' memory in a delayed-match-to-sample task using an egocentric (body-turn) response in a cross-maze; damage to the AT left performance intact (Mitchell and Dalrymple-Alford, 2006). Further studies from the same laboratory have provided more insight into the dissociable effects of lesions to the AT and lateral MD combined with ILN lesions in learning and memory processing (Gibb et al., 2006; Wolff et al., 2008; Lopez et al., 2009; Moreau et al., 2013). These authors concluded from this and the above evidence that no single medial thalamic structure is critical for all of the memory and other cognitive deficits associated with thalamic amnesia. Instead many subdivisions of medial thalamic nuclei are contributing to independent neural networks via subcortical and cortical interactions and are integrating information for successful cognition (Mitchell and Dalrymple-Alford, 2005, 2006). Other research (e.g., Eleore et al., 2011) has also documented similar roles for other thalamic nuclei, namely the reuniens, in supporting the acquisition of associative learning using a classical eyeblink conditioning task with a trace paradigm, because high frequency train stimulation directed at the reuniens in behaving mice prevented the proper acquisition of the task.

The experiment demonstrating bilateral MD involvement in recency memory (Mitchell and Dalrymple-Alford, 2005) has been further confirmed by Cross et al. (2012). The medial PFC (mPFC) is involved in recency memory processing in rodents (Mitchell and Laiacona, 1998; Hannesson et al., 2004; Cross et al., 2012) and patients with Korsakoff's syndrome and frontal lobe damage have problems with temporal processing and recency discriminations (Kopelman et al., 1997; Kopelman, 2002; Fuster, 2008).

Cross et al. (2012) have demonstrated in rodents the importance of neural communication in MD-mPFC circuitry for successful recency recognition memory. After combining crossed unilateral lesions of the MD and mPFC, a lesion that disconnects the structures in both hemispheres, animals were impaired in the recency recognition memory task. In contrast, after a combined ipsilateral unilateral lesion of the MD and mPFC (essentially a control lesion of these two structures that leaves one hemisphere functioning) recency recognition memory performance was left intact. The authors proposed that “during associative or recency recognition memory tasks, the MD–mPFC connection might be necessary to direct ongoing behavior toward, for example, the novel object–place configuration” (Cross et al., 2012). This study highlights how the interplay of communication within and between MD-mPFC networks is clearly critical for cognition.

After neurotoxic lesions to MD, rodents are not impaired at SOR tasks (Hunt and Aggleton, 1998a; Mitchell and Dalrymple-Alford, 2005; Cross et al., 2012). As already noted above, conflicting evidence exists for the role of MD in various recognition memory tests. Several studies have reported deficits in object recognition using rats involving delay non-matching-to-sample tasks that involve object-reward associations (Hunt and Aggleton, 1991; Mumby et al., 1993), while others have reported no deficits on various unrewarded recognition memory tasks that rely on spontaneous exploration instead (M'Harzi et al., 1991; Hunt and Aggleton, 1998b; Kornecook et al., 1999; Mitchell and Dalrymple-Alford, 2005). The most parsimonious explanation is that the task demands related to reward are different, as SOR does not involve reward but rather relies on spontaneous exploration while delayed-matching or non-matching to sample tasks normally reward the animal for a correct response, thus engaging associative memory networks instead (Parker et al., 1997; Gaffan and Parker, 2000). In addition, it is now known that pre-operative training is a critical factor in learning and memory tasks, as damage to the MD does not impair retention of pre-operatively acquired information associating objects and rewards (Gaffan and Parker, 2000; Mitchell and Gaffan, 2008). Also, there may be cross species differences in neuroanatomy. That is, in macaques there is a distinct projection from the rhinal cortices (perirhinal and entorhinal) to the MDmc (Aggleton et al., 1986; Saunders et al., 2005), while a similar projection is not as robust in rodents (Burwell et al., 1995).

As in monkey studies, researchers have investigated the devaluation effects after bilateral MD lesions in rodents. Pickens (2008) has systematically assessed rats with MD lesions on many variants of devaluation testing using Pavlovian and operant contingencies, and single and multiple reinforcement paradigms. Pickens concluded from this series of experiments, that the MD is important in devaluation circuits only “in cases in which previous associations need to be suppressed in order for new associations to be learned and control behavior, otherwise the devaluation circuit does not require MD” (Pickens, 2008).

Thus, through experimental testing in both rats and non-human primates it has been shown that the different subdivisions of the MD provide critical contributions to successful cognitive processing in many different tasks. Principally, the MD in conjunction with its neuroanatomical connections is important for some forms of recognition memory, recency memory processing, and further prospective integration of the rewards associated with successful responses to govern additional responses, as well as new learning of OIP discriminations, but not their retention. The subdivisions of the MD provide key roles in helping integrate object/reward/response information for successful new learning and successful additional (future) responding. Furthermore, and most importantly, it has been demonstrated that the MD contributes to successful cognition, rather than causing memory and other cognitive deficits by simply causing a generalized dysfunction of the PFC.