Paul M. Gochin

FUNCTIONAL INTERACTIONS AMONG NEURONS IN INFERIOR TEMPORAL CORTEX OF THE AWAKE MACAQUE

SummaryFunctional interactions among inferior temporal cortex (IT) neurons were studied in the awake, fixating macaque monkey during the presentation of visual stimuli. Extracellular recordings were obtained simultaneously from several microelectrodes, and in many cases, spike trains from more than one neuron were extracted from each electrode by the use of spike shape sorting technology. Functional interactions between pairs of neurons were measured using cross-correlation. Discharge patterns of single neurons were evaluated using auto-correlation and PST histograms. Neurons recorded on the same electrode (within about 100 μn) had more similar stimulus selectivity and were more likely to show functional interactions than those recorded on different electrodes spaced about 250 to 500 microns apart. Most neurons tended to fire in bursts tens to hundreds of milliseconds in duration, and asynchronously from the stimulus induced rate changes. Correlated neuronal firing indicative of shared inputs and direct interactions was observed. Occurrence of shared input was significantly lower for neuron pairs recorded on different electrodes than for neurons recorded on the same electrode. Direct connections occurred about as often for neurons on different electrodes as for neurons on the same electrode. These results suggest that input projections are usually restricted to less than 500 μm patches and are then distributed over greater distances by intrinsic connections. Measurements of synaptic contribution suggest that typically more than 5 near-simultaneous inputs are required to cause an IT neuron to discharge.

Habituation-like decrease in the responses of neurons in inferior temporal cortex of the macaque

In both anesthetized and behaving macaques, we examined the responses of neurons in the inferior temporal cortex (IT) to repeated presentation of a visual stimulus. In anesthetized animals, the responsiveness of IT neurons decreased with repeated stimulus presentation at interstimulus intervals (ISIs) of 2-12 s but not at 20 s. Responsiveness recovered after a 5-min period of no stimulus presentation. The response decrement was similar in anesthetized and awake animals at a 2-s ISI, but at a 6-s ISI, response decrement in the awake animal was much less.

Coordinated activity of neuron pairs in anesthetized rat dorsal cochlear nucleus

We have recorded from small groups of neurons in the dorsal cochlear nucleus of anesthetized rats in an effort to study neuronal interactions. Multi-unit recordings on each single electrode were sorted by waveform into spike trains from individual neurons using a principal components spike sorter. Pairs of such sorted spike trains were studied with cross-correlation analysis to detect excitatory and/or inhibitory interactions. In a few cases recordings were obtained from two electrodes simultaneously, thus allowing cross-correlation studies without the consequences of spike train waveform sorting. All neurons were characterized by their strongest response frequency (at a fixed sound pressure level) and peristimulus histogram responses to 55 ms tone bursts. Fifty-eight percent of the neuron pairs studied showed peaks in their cross-correlograms indicative of coordinated neural activity. Of these pairs, 86% showed peak configurations (i.e. correlograms with asymmetrically located peaks) consistent with the interpretation that one cell induced the other to discharge. The remaining correlograms contained symmetric peaks which were centrally located, possibly due to shared input to these neuron pairs. Latencies of asymmetric peaks in cross-correlograms were typically 2 ms; consequently, an intervening excitatory synapse may be involved. Similar results were obtained from at least one pair of neurons where each neuron was recorded by a separate electrode. Strongest response frequencies of each neuron pair, for which they could be determined, were within 0.17 log units. Peristimulus histograms from each neuron in these pairs revealed that it was common for adjacent cells to respond with differing time patterns under the same stimulus conditions. The variations in histogram patterns of interconnected neurons suggests some relatively complex integrative function for these circuits.

Neuronal Population Coding and the Elephant

We examine several ways to study representation of external events by a population of neurons. It is useful in the laboratory to consider this as a clustering problem, but it is not clear whether and how the brain accomplishes a similar result. In particular there is a problem in selecting which neurons are to be included in the population analysis.

Dynamic temporal properties of effective connections in rat dorsal cochlear nucleus

In a prior report we presented evidence that functionally connected dorsal cochlear nucleus (DCN) neurons in close proximity can show differing peristimulus time histograms (PSTHs) in response to the same stimulus. We wished to further investigate how interconnections between such neurons might participate in the PSTH patterns. Methodology has recently been developed which permits measurement of rapid changes in effective connectivity between neuron pairs: the normalized joint PSTH. Using this technique we have observed that rapid changes in effective connectivity do occur in the DCN. These observations demonstrate that the effects of one DCN neuron on another cannot necessarily be understood by sequential recordings from single units, even if anatomical connectivity can be established.

A hierarchical machine vision system based on a model of the primate visual system

The problem of advancing machine visual pattern recognition capabilities is approached by examining the visual system of the primate. A model of biological vision is suggested, and an analogous machine vision simulation is developed. The modeling is limited to luminance information (color, motion, and depth are not considered), and biological systems are considered at the network level (biochemical and biophysical details are not simulated). The system architecture consists of a set of invariance transforms (luminance, spatial, and scale) followed by storage using an adaptive resonance theory network.<<ETX>>