[Abstract Title]. - Society for Neuroscience

Program#/Poster#: 296.2/TT8
Topic: F.02.i. Learning and memory: Physiology and imaging
Support: NIH Grant MH058847
Title: Neuronal activity encoding temporal-order memory of visual objects in the macaque
entorhinal and perirhinal cortices
Authors: *Y. NAYA, W. A. SUZUKI;
Ctr. Neural Sci., New York Univ., New York, NY
Abstract: The ability to recall specific temporal sequences of events in an episode is severely
impaired in humans with memory impairment following medial temporal lobe damage. In the
present study, we characterize neural activity in the macaque entorhinal and perirhinal (ER &
PR) cortices as animals processed temporal order of items in memory. The task includes both an
encoding and a retrieval phase. The encoding phase starts with the animal fixating a fixation
point on a video monitor for 0.4 s. Then, a sequence of two cue stimuli (0.3 s for each) was
presented with a blank delay interval (0.9 s) between them. Cue stimuli were pseudo-randomly
chosen out of a pool of eight well-learned visual items. Following the second cue stimulus,
another 0.5 s of fixation was required for the animal and a single drop of water was given as a
reward. Then a blank inter-phase interval of 0.7-1.5 s was presented during which time no
fixation was required. The retrieval phase started with the animal fixating a fixation for 1.0 s.
Then, three choice stimuli were presented together; two of them were the items that had been
presented as cue stimuli in the encoding phase, and another was chosen from the other six items.
If the animal touched the two items in the same order as they were presented in the encoding
phase, six drops of water was given as a reward. We recorded 125 neurons from the ER and PR
cortices: 71 from Monkey G and 54 from Monkey B. Their responses during the cue periods
were characterized at the single neuron level using a two-way ANOVA with temporal rank order
and stimulus identity as main factors. We found 19.2 % (n = 24) of the recorded neurons showed
temporal-order-selective responses (P < 0.05) and 36.8 % (n = 46) showed stimulus-selective
responses (P < 0.05). Moreover, 19.6 % (n = 9) of stimulus-selective neurons showed significant
interaction between temporal order and stimulus identity (P < 0.05). These results suggest that
ER and PR neurons as a whole represent rank order information as well as visual object
information during the temporal order task and that some neurons can encode a particular
temporal order of specific items at the single neuron level. This latter finding cannot be
explained by a simple linear summation of temporal-order-selective responses and stimulusselective
responses. Taken together, our present study suggests that ER and PR cortices
participate in the encoding of stimulus-selective information for a temporal order memory task.
Disclosures: Y. Naya, None; W.A. Suzuki, None.
Poster