Abstract
Inhibitory projections from the visual sector of the thalamic reticular nucleus to the lateral geniculate nucleus complete the earliest feedback loop in the mammalian visual pathway and regulate the flow of information from retina to cortex. There are two competing hypotheses about the function of the thalamic reticular nucleus. One regards the structure as a thermostat that uniformly regulates thalamic activity through negative feedback. Alternatively, the searchlight hypothesis argues for a role in focal attentional modulation through positive feedback, consistent with observations that behavioral state influences reticular activity. Here, we address the question of whether cells in the reticular nucleus have receptive fields small enough to provide localized feedback by devising methods to quantify the size of these fields across visual space. Our results show that reticular neurons in the cat operate over discrete spatial scales, at once supporting the searchlight hypothesis and a role in feature selective sensory processing.
Highlights
Inhibitory projections from the visual sector of the thalamic reticular nucleus to the lateral geniculate nucleus complete the earliest feedback loop in the mammalian visual pathway and regulate the flow of information from retina to cortex
The fields were mapped using standard methods of reverse correlation and the resulting spike-triggered averages (STAs) of the stimulus ensemble are displayed as a sequence of contour plots that show the temporal evolution of the response (Fig. 2a–c); the stimulus was a sequence of checkerboards in which the luminance of each component pixel was independently drawn from a Gaussian distribution
The searchlight hypothesis assumes that neurons in the thalamic reticular nucleus (TRN) are able to exert a focal influence on the lateral geniculate nucleus (LGN), whereas the thermostat hypothesis supposes a widespread effect[1]
Summary
Inhibitory projections from the visual sector of the thalamic reticular nucleus to the lateral geniculate nucleus complete the earliest feedback loop in the mammalian visual pathway and regulate the flow of information from retina to cortex. The searchlight hypothesis argues for a role in focal attentional modulation through positive feedback, consistent with observations that behavioral state influences reticular activity. We address the question of whether cells in the reticular nucleus have receptive fields small enough to provide localized feedback by devising methods to quantify the size of these fields across visual space. The PGN senses distributed activity levels in the LGN, exerting negative feedback; to quote Crick, “the function of the reticular complex would be to act as an overall thermostat of thalamic activity”[1]; we refer to this as the thermostat hypothesis (Fig. 1e). The top-down signals that encode spatial attention should modulate activity in the PGN This descending input should increase the activity of relay cells whose receptive fields fall within the area of interest. The third prediction, which is the focus of the current study, is important to sensory processing per se as it suggests that reticular input has a role in processing spatially discrete visual features
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