Indirect inputs to ventral temporal cortex of monkey: the influence of unit activity of alerting auditory input,...

1. This study examined nonvisual and indirect inputs to 1,021 single units recorded in inferotemporal and parahippocampal cortex of behaving macaques. 2. To better isolate these influences, a fully split-brain, split-chiasm preparation was used. Extracellular single-unit activity was recorded while the ipsilateral eye was covered. During the recordings the monkeys worked on a visual discrimination task that consisted of a series of presentations of single images. 3. When the interval between presentations was varied randomly (usually between 4 and 15 s) about one-quarter of these cells responded to an alerting tone sounded 500 ms before the onset of the visual image. That this response is due to the warning value of the tone was shown by finding that an identical tone sounded at the end of each trial produced no response from these cells. Use of an exchange between pairs of light-emitting diodes as a warning signal (one turned on as the other was turned off, also 500 ms before the visual stimulus onset) produced a similar response in many units. This indicates a subcortical route for the alerting signal. In most cases, warning responses were inhibitory, often delayed with respect to the warning signal occurrence to more nearly match the image arrival time. 4. Surprisingly, and despite the monkeys' confirmed split-brain status, occasional cells (approximately 2%) showed a response from a visual presentation limited to the other hemisphere. Although this subcortical visual input was far weaker than direct visual input, it was nonetheless statistically reliable. Importantly, the indirect input was stimulus specific and could form the neural basis for a limited interhemispheric visual transfer of the sort seen in human split-brain patients. 5. Also rarely, cells showed activity time locked to the animal's behavioral response.