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The Function of Bursts of Spikes During Visual Fixation in the Awake Primate Lateral Geniculate Nucleus and Primary Visual Cortex

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The Function of Bursts of Spikes During Visual Fixation in the Awake Primate Lateral Geniculate Nucleus and Primary Visual Cortex The function of bursts of spikes during visual fixation in the awake primate lateral geniculate nucleus and primary visual cortex Susana Martinez-Conde*†‡§, Stephen L. Macknik*†‡, and David H. Hubel* *Department of Neurobiology, Harvard Medical School, 220 Longwood Avenue, Boston, MA 02115; and †Department of Visual Science, Institute of Ophthalmology, University College London, 11-43 Bath Street, London EC1V 9EL, United Kingdom Contributed by David H. Hubel, August 19, 2002 When images are stabilized on the retina, visual perception fades. Methods During voluntary visual fixation, however, constantly occurring We recorded from neurons in the LGN and area V-1 of awake small eye movements, including microsaccades, prevent this fad- rhesus macaques. Before the experiments began, the monkeys ing. We previously showed that microsaccades generated bursty were implanted with a head post, recording chamber, and a firing in the primary visual cortex (area V-1) in the presence of scleral eye coil. Standard sterile surgical techniques and animal stationary stimuli. Here we examine the neural activity generated care methods were used (9). The Harvard Medical Area Stand- by microsaccades in the lateral geniculate nucleus (LGN), and in the ing Committee on Animals approved all surgical and electro- area V-1 of the awake monkey, for various functionally relevant physiological methods. stimulus parameters. During visual fixation, microsaccades drove We trained the monkeys to fixate their gaze on a small cross LGN neurons by moving their receptive fields across a stationary within a 2° window in exchange for fruit juice (eye movements stimulus, offering a likely explanation of how microsaccades block exceeding the limits of the fixation window were also recorded). fading during normal fixation. Bursts of spikes in the LGN and area Single units were recorded extracellularly with lacquer-coated V-1 were associated more closely than lone spikes with preceding electropolished tungsten electrodes (13). At the beginning of microsaccades, suggesting that bursts are more reliable than are each LGN-recording session we lowered the electrode into the lone spikes as neural signals for visibility. In area V-1, microsaccade- brain while flashing a full-field stimulus to drive the LGN generated activity, and the number of spikes per burst, was responses. We identified the LGN location from stereotaxic maximal when the bar stimulus centered over a receptive field coordinates and from the physiological properties of single units. matched the cell’s optimal orientation. This suggested burst size as Before the V-1 recordings we removed a small portion of the a neural code for stimuli optimality (and not solely stimuli visibil- dura mater. After isolating each single LGN or V-1 cell we mapped its receptive field and determined the optimal width of ity). As expected, burst size did not vary with stimulus orientation the bar stimulus. In the V-1 cells, we also determined the in the LGN. To address the effectiveness of microsaccades in preferred orientation by recording the responses to oriented generating neural activity, we compared activity correlated with bars. (All orientations were represented in 10-degree steps.) microsaccades to activity correlated with flashing bars. Onset Light bar stimuli had a luminance of 24.3 cd͞m2, and the dark responses to flashes were about 7 times larger than the responses monitor background was 3.8 cd͞m2. (These values were opposite to the same stimulus moved across the cells’ receptive fields by for dark bars over light backgrounds.) Most eye movements microsaccades, perhaps because of the relative abruptness of during fixation were about the same or larger (Ϸ0.33°) than flashes. receptive field sizes in the LGN and area V-1, so the bars were sometimes located over the receptive fields centers, and some- hen the visual world is stabilized on the retina, visual times not. Neuronal eccentricities ranged from 1° to 30°. Eye Wperception fades as a consequence of neural adaptation movements and spikes were sampled at 1 kHz in consecutive 2-s (1–4). But during normal vision we move our eyes involuntarily trials. The beginning and end of each 2-s sampling period were every few hundred milliseconds, even as we try to fixate our gaze unknown to the animal. The bar turned on over the receptive on a small stimulus, preventing retinal stabilization and the field before data collection, and both the bar and the fixation associated fading of visibility. These fixational eye movements cross persisted between trials. We identified microsaccades include ‘‘microsaccades,’’ small ballistic unidirectional eye move- automatically with a computer algorithm as described (9). To ments that are generated at random intervals in all directions. distinguish microsaccades from small artifacts and larger eye Fixational eye movements, including microsaccades, have been movements we applied lower and upper limits of 3 arcmin and correlated with stimulus visibility (5–8), and in a previous paper 2° to the size of microsaccades. We considered a microsaccade completed when its speed dropped below 3°͞s or its direction we showed that microsaccades increase the probability of firing changed by more than 15°. in area V-1 cells by moving their receptive fields over stationary We established in a previous article (9) that bursts of spikes are stimuli (9). Here we ask whether microsaccades might also better correlated with microsaccades (and therefore with visi- induce an increase in neural activity at an earlier level, in the bility) than either single spikes or instantaneous firing rate. To neurons of the lateral geniculate nucleus (LGN). We also ask determine the spike-grouping parameters that most optimally how effective microsaccades are in generating neural activity by defined bursts, we used the correlation between the various spike comparing them with previously characterized and well known groupings and microsaccades as a guide; we let perception visual stimuli, flashing bars. Finally, because transient neural (rather than biophysics) show us what constituted a burst. That firing (i.e., bursts of spikes) has been proposed as a neural code is, because microsaccades are correlated with perception, by for the visibility of a stimulus (9–12), we also ask here whether bursts of spikes are used by the visual system to encode the salience of a stimulus. To answer this question, we compare the Abbreviations: LGN, lateral geniculate nucleus; ISI, interspike interval. neural activity generated by microsaccades in the presence of ‡S.M.-C. and S.L.M. contributed equally to this work. different stimulus orientations, in both area V-1 and the LGN. §To whom correspondence should be addressed. E-mail: [email protected]. 13920–13925 ͉ PNAS ͉ October 15, 2002 ͉ vol. 99 ͉ no. 21 www.pnas.org͞cgi͞doi͞10.1073͞pnas.212500599 Downloaded by guest on September 24, 2021 determining optimal parameters for grouping spikes into bursts as a function of their correlation with previous microsaccades, we let perception itself determine what a meaningful burst is. (See ref. 9 for a complete description of the burst analysis.) The factor most important to grouping spikes into bursts is the interspike interval (ISI). Thus, to measure the burst parameters that best correlated with microsaccades, we measured the ISI between all spikes in the spike train and assigned each spike uniquely to bursts of different sizes as a function of the ISI. Thus, two spikes with a given ISI (or less) were considered part of the same burst, and two spikes separated by an interval greater than that given ISI were considered to be part of two separate bursts. A burst of one spike (or a lone spike) was therefore a single spike preceded and followed by time intervals greater than the given ISI. A burst that was greater than one spike in length had its first spike at least one ISI after the previous spike, its last spike at least one ISI before the subsequent spike, and all of its internal Fig. 1. Comparison between microsaccades and flashes. (A) Microsaccades spikes within one ISI of each other. Because the tested neurons increase spike probabilities in the presence of a stationary bar in the LGN (thick pink trace; n ϭ 57 neurons) and area V-1 (thick black trace; n ϭ 308 neurons). may have had different morphologies, inputs, and biophysical The correlation between microsaccades and spikes disappears in the absence properties, the burst parameters (optimal ISI, latency, and burst of visual stimulation in the LGN (thin pink trace; n ϭ 42 neurons) and V-1 (thin size) were necessarily determined individually for each neuron gray trace; n ϭ 37 neurons). Microsaccades increase spike probabilities in the (instead of choosing arbitrary values for the whole population). LGN (thick purple trace; n ϭ 48 neurons) and area V-1 (thick gray trace; n ϭ 6 Because we could not know a priori which ISI was optimal for a neurons) when a flashing bar is on. Starts of all microsaccades are aligned at given cell, we determined the optimal ISI by recalculating the the vertical line. (B) The probability of a spike after a flashing bar turns on is burst analysis 100 times by using each ISI between 1 and 100 ms. Ϸ7 times higher than the probability of a spike after a microsaccade when that We then measured the peak magnitude of the correlation (for same flashing bar is on. The same data set from A (LGN and V-1: purple and each of the 100 sets of bursts) between each burst and the black traces) have been replotted and realigned to the flashing bar onset (vertical line). presence of microsaccades, for latencies between 1 and 200 ms before the first spike of the burst. The ISI that resulted in bursts with the best correlation with previous microsaccades was thus receptive field of the neuron (but left the fixation point in place, the best ISI a postsynaptic neuron could use to determine the so the monkey could still fixate and make microsaccades).
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