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Monday, July 20 • 7:00pm - 8:00pm
P220: Extracellular positive spikes in cat primary visual cortex may correspond from the axons of cells originating from the thalamus

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Zoom: https://unimelb.zoom.us/j/94143193969?pwd=QkJoa0IwTm9xaU5BOWhPRlVzN0l0QT09
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Shi Hai Sun
, Ali Almasi, Hamish Meffin, Michael R Ibbotson, Molis Yunzab

Extracellular spike waveforms from recordings in the visual cortex have been classified into either regular spiking (RS) or fast spiking (FS) units, which are commonly associated with excitatory and inhibitory neurons, respectively. While both these types of spike waveforms are negative-dominant, we show that there are also distinct classes with positive-dominant waveforms, which are not regularly reported. The spatial receptive fields (RFs) of these different spike waveform types were estimated and we found that each spike type had distinctly different RF structure.

In this study, we systemically classified 1,225 single units (SUs) in cat visual area 17 (V1) into five categories by the shape of their spike waveforms: RS units (53%, n =645) which are biphasic, have a dominant negative peak, and a slow declining slope at the end of the waveform; FS units (18%, n = 226) which are biphasic, have a dominant negative peak, and a fast declining slope at the end of the waveform; triphasic spiking units (TS, 10%, n = 122) which have a positive first peak that is >10% of the negative peak, followed by a large negative peak and then a smaller positive peak; compound spiking units (CS, 7%, n = 82) which are also triphasic but with a significantly longer waveform; and positive spiking units (PS, 12%, n = 150) which have a positive peak greater than the negative peak.

Of these 1,225 SUs, 341 had their spatial RFs estimated as the spatial filters in a powerful model-based analysis method to objectively determine the RF characteristics of the recorded units, which revealed the existence of non- oriented and blob-like (orientation bandwidth > 110°) and oriented and Gabor- like RFs (orientation bandwidth < 90°). RS and FS units had mostly oriented RFs (94%, and 96%, respectively), TS and CS units have an even mixture of both RF types (47% oriented and 53% non-oriented, and 56% oriented and 44% non- oriented, respectively), while PS units had mostly non-oriented RFs (83% blob- like).

Units with non-oriented RFs have similar spatial structures to the centre- surround RFs reported in the thalamus, suggesting that units with non-oriented RFs could have originated from the sub-cortical area. We calculated several response properties that are statistically distinguishable between cortical and thalamic neural populations: spike-rate, burstiness, and response latency. On average, PS units had significantly higher spike-rate (t-test, p < 0.01), significantly higher proportion of burst spikes (p < 0.001), and significantly shorter response latency (p < 0.001) to RS and FS units. We also recorded from V1 before and after the application of muscimol (a cortical silencer) and found that PS units remained while RS and FS units did not.

Thus, our results suggest that PS units, which have mostly non-oriented RFs, thalamic-like response properties and remain after cortical silencing, are recordings of axons originating from the thalamus. RS and FS units correspond to cortical neurons, which have mostly orientated RFs and do not remain after cortical silencing. Our results suggest that cortically implanted electrodes are able to record activity simultaneously from thalamic axon afferents and from the somas of cortical neurons, thus allowing us to assess connectivity between two brain areas while only recording from one area.

Speakers
SH

Shi Hai Sun

National Vision Research Institute, Melbourne



Monday July 20, 2020 7:00pm - 8:00pm CEST
Slot 01