Interfering with Fos expression in rat perirhinal cortex impairs recognition memory.

Previous work has shown that immunohistochemical imaging of Fos protein is a reliable marker for changes in activity related to recognition memory in the perirhinal (PRH) cortex of the medial temporal lobe; however, whether PRH Fos expression is necessary for recognition memory had not been established. To investigate this potential requirement, antisense Fos oligodeoxynucleotide (ODN) was infused locally into PRH cortex to interfere with Fos production. As in previous studies, differential Fos expression produced by viewing novel or familiar visual stimuli was measured by immunohistochemistry: antisense Fos ODN infusion into PRH cortex disrupted the normal pattern of differential Fos expression in PRH cortex. The effect of antisense Fos ODN infusion into PRH cortex was therefore sought on recognition memory. Infusion before or immediately after acquisition impaired recognition memory for objects when the memory delay was 3 or 24 h, but not when the delay was 20 min, or when the ODN was infused before retrieval after a 24-h delay. The findings indicate a role for Fos in consolidation processes underlying long-term recognition memory for objects and establish that interfering with its expression impairs recognition memory. Antisense Fos ODN infusion also impaired object-in-place recognition memory. The results demonstrate that Fos is necessary for neuronal mechanisms in PRH cortex essential to recognition memory.

Spatial distribution of suppressive signals outside the classical receptive field in lateral geniculate nucleus.

A suppressive surround modulates the responsiveness of cells in the lateral geniculate nucleus (LGN), but we know nothing of its spatial structure or the way in which it combines signals arising from different locations. It is generally assumed that suppressive signals are either uniformly distributed or balanced in opposing regions outside the receptive field. Here, we examine the spatial distribution and summation of suppressive signals outside the receptive field in extracellular recordings from 46 LGN cells in anesthetized marmosets. The receptive field of each cell was stimulated with a drifting sinusoidal grating of the preferred size and spatial and temporal frequency; we probed different positions in the suppressive surround with either a large half-annular grating or a small circular grating patch of the preferred spatial and temporal frequency. In many of the cells with a strong suppressive surround (29/46), the spatial distribution of suppression showed clear deviation from circular symmetry. In the majority of these of cells, suppressive signals were spatially asymmetrical or balanced in opposing areas outside the receptive field. A suppressive area was larger than the classical receptive field itself and spatial summation within and between these areas was nonlinear. There was no bias for suppression to arise from foveal or nasal retina where cone density is higher and no other sign of a systematic spatial organization to the suppressive surround. We conclude that nonclassical suppressive signals in LGN deviate from circular symmetry and are nonlinearly combined.

Contribution of the primate prefrontal cortex to the gap effect.

The introduction of a brief temporal gap between the disappearance of the initial fixation point and the presentation of a peripheral target leads to a general reduction in saccadic reaction times (SRTs), known as the gap effect. Moreover, extremely short latency express saccades frequently occur in this paradigm. Disorders of the prefrontal cortex (PFC) are often associated with increased numbers of express saccades and an inability to suppress reflexive saccades. To investigate the role of the PFC in the gap effect and in express saccade generation, we trained two rhesus monkeys on a gap saccade task in which the initial fixation point (FP) disappeared 200 ms or 600 ms before a peripheral stimulus appeared either 8 degrees to its left or right side. We recorded from the lateral PFC (areas 8 Ar and 46) in both monkeys the activity of 214 neurons, 84 (39%) of which exhibited task-related activity. These neurons could be further categorized into separate groups based on their discharge behaviour: fixation neurons with a decrease in activity during the gap (27%), FP offset neurons (12%), preparatory neurons with an increase in activity during the gap (30%), visual neurons (6%), post-saccadic neurons (8%), and reward-related neurons (12%). There were no obvious differences in the topography of these groups. Significant differences between express and regular saccade trials were found for fixation-related neurons. These neurons had a lower activity during the gap prior to the generation of contralateral express saccades. We hypothesize that a reduction in the activity of fixation-related neurons in the PFC may contribute to the elevated rate of express saccades in prefrontal disorders.