Mammilliothalamic tract lesions disrupt tests of visuo-spatial memory.

Abstract

The mammillary bodies and their projections via the mammilliothalamic tract to the anterior thalamic nuclei are known to be important for spatial memory in rodents, but their precise role remains unclear. To determine whether transection of the mammilliothalamic tract can produce deficits on tests of spatial memory even when the navigational demands placed on the animal are limited, rats with discrete mammilliothalamic tract lesions were tested on the ability to use distal visual cues to discriminate between 2 locations within a room, irrespective of the direction traveled (Experiment 1). Animals with mammilliothalamic tract lesions acquired this task more slowly and less accurately than control animals. Consistent with this impairment in discriminating different spatial locations, the same lesions also severely disrupted object-in-place memory but spared performance on standard tests of object recognition memory (Experiment 2). Finally, to compare performance on a task that is known to be sensitive to mammilliothalamic transection and requires animals to actively navigate within their environment, the effect of the lesions on spatial working memory in the radial-arm maze was examined. Taken together, the results suggest that even when there are little or no navigational demands, mammilliothalamic tract damage still results in impoverished encoding of spatial location.

Mammillothalamic disconnection alters hippocampo-cortical oscillatory activity and microstructure: Implications for diencephalic amnesia

“…This, in addition to Nissl staining, provides further verification of lesion accuracy. Consistent with previous studies (Nelson and Vann, 2014;Vann, 2013), MTT lesion animals (Fig. 1C, D) exhibited significant spatial memory impairments both on a radial-arm maze task (Experiment 3/Cohort 4; F(1, 18) = 14.08, p = 0.001; Fig.…”

Anterior thalamic nuclei lesions have a greater impact than mammillothalamic tract lesions on the extended hippocampal system

“…Previously, only separate studies have been available to estimate the comparative effects of MTT and ATN lesions on memory tasks. The similar effects of both lesion types have been emphasized, in that neither lesion impairs novel object discrimination (Moran & Dalrymple‐Alford, ; Nelson & Vann, ; Warburton & Aggleton, ), but both lesions impair temporal memory for a sequence of items (Dumont & Aggleton, ; Nelson & Vann, ; Wolff et al, ,b) and both lesions impair spatial working memory in radial‐arm maze tasks (Aggleton, Hunt, Nagle, & Neave, ; Harland, Collings, McNaughton, Abraham, & Dalrymple‐Alford, ; Mitchell & Dalrymple‐Alford, ; Nelson & Vann, ; Sziklas & Petrides, ; Vann, ; Vann & Aggleton, ). The greater severity of spatial working memory deficits after ATN lesions reported here is reminiscent of the T‐maze alternation deficits found previously across separate ATN or MTT lesion studies.…”

How do mammillary body inputs contribute to anterior thalamic function?

“…The radial-arm maze task requires animals to retrieve rewards from the arms of the maze without re-entering the same arm; effective performance requires the animal to monitor the arms it has already entered. Transection of the mammillothalamic tract reliably produces equivalent deficits to mammillary body lesions on both of these tests of spatial working memory ( Field et al, 1978; Nelson and Vann, 2014; Vann and Aggleton, 2003; Vann, 2013 ). The effects of mammillary body lesions on spatial learning in the water-maze have also been examined.…”

“…For example, mammillothalamic tract lesions do not disrupt geometric learning in the water-maze (using the shape of the maze to find the hidden platform) ( Vann, 2013 ) and reference memory in the water-maze can be unaffected by mammillary body lesions ( Santín et al, 1999 ). Conversely, lesion deficits emerge on tasks which contain little or no navigational component ( Nelson and Vann, 2014 ). For example, mammillothalamic tract lesions disrupt the ability to use distal visual cues to discriminate between two locations within a room, irrespective of the direction travelled ( Nelson and Vann, 2014 ).…”

“…Conversely, lesion deficits emerge on tasks which contain little or no navigational component ( Nelson and Vann, 2014 ). For example, mammillothalamic tract lesions disrupt the ability to use distal visual cues to discriminate between two locations within a room, irrespective of the direction travelled ( Nelson and Vann, 2014 ). Rather, it would appear that mammillary body and mammillothalamic tract lesions produce deficits when animals are required to use distal spatial cues or the rapid encoding of new spatial information is required.…”