Spatial organization of receptive fields of V1 neurons of alert monkeys: comparison with responses to gratings

of receptive fields of V1 neurons of alert monkeys: comparison with responses to gratings. J Neurophysiol 88: 2557–2574, 2002; 10.1152/jn.00858.2001. We studied the spatial organization of receptive fields and the responses to gratings of neurons in parafoveal V1 of alert monkeys. Activating regions (ARs) of 228 cells were mapped with increment and decrement bars while compensating for fixational eye movements. For cells with two or more ARs, the overlap between ARs responsive to increments (INC) and ARs responsive to decrements (DEC) was characterized by a quantitative overlap index (OI). The distribution of overlap indices was bimodal. The larger group (78 % of cells) was composed of complex cells with strongly overlapping ARs (OI � 0.5). The smaller group (14%) was composed of simple cells with minimal spatial overlap of ARs (OI � 0.3). Simple cells were preferentially located in layers dominated by the magnocellular pathway. A third group of neurons, the monocontrast cell

Visual Psychophysics and Physiological Optics Enhancing Performance While Avoiding Damage: A Contribution of Macular Pigment

PURPOSE. To compare action spectra for visual discomfort in the fovea and the parafovea and to determine the effect of macular pigment (MP). METHODS. Visual discomfort thresholds to lights from 440 to 600 nm were obtained for six young (<35 y), visually normal subjects with a wide range of MP densities (0.10-0.71 at 30 0 eccentricity). Foveal and parafoveal conditions were assessed. Discomfort thresholds were also obtained for xenon-white light (partially absorbed by MP), and a broadband yellow (outside the absorption band of MP). MP was measured psychophysically using heterochromatic flicker photometry (HFP). RESULTS. For the parafovea, discomfort sensitivity (1/threshold) increased sharply with decreasing wavelength for all subjects. Commensurate with a subject's MP level, MP significantly reduced visual discomfort to short wavelengths (including xenon-white light) for central viewing. CONCLUSIONS. MP simultaneously reduces visual discomfort and protects from light damage at short wavelengths. As a result, MP increases the range of safe and comfortable light levels. Because higher light levels enable improved visual sensitivity for fine detail, these findings indicate that the spectral absorption properties and spatial distribution of MP combine to protect the retina while enhancing visual performance. The action spectrum for visual discomfort closely matches the risk for acute light damage to the retinal pigment epithelium, and it is consistent with a major influence from the intrinsically photosensitive retinal ganglion cells containing melanopsin. We suggest that MP interacts with nonimage-forming retinal input to achieve the dual outcomes of visual discomfort reduction and protection from light damage

Orientation and direction selectivity of neurons in V1 of alert monkeys: functional relationships and laminar distributions.

Abstract We studied orientation selectivity in V1 of alert monkeys and its relationship to other physiological parameters and to anatomical organization. Single neurons were stimulated with drifting bars or with sinusoidal gratings while compensating for eye position. Orientation selectivity based on spike counts was quantified by circular variance and by the bandwidth of the orientation tuning curve. The circular variance distribution was bimodal, suggesting groups with low and with high selectivity. Orientation selectivity was clearly correlated with spontaneous activity, classical receptive field (CRF) size, and the strength of surround suppression. Laminar distributions of neuronal properties were distinct. Neurons in the output layers 2/3, 4B and 5 had low spontaneous activity, small CRFs, and high orientation selectivity, while the input layers had greater diversity. Direction-selective cells were among the neurons most selective for orientation and most had small CRFs. A narrow band of direction-and orientation-selective cells with small CRFs was located in the middle of layer 4C, indicating appearance of very selective cells at an early stage of cortical processing. We suggest that these results reflect interactions between excitatory and inhibitory mechanisms specific to each sublamina. Regions with less inhibition have higher spontaneous activity, larger CRFs, and broader orientation tuning. Where inhibition is stronger, spontaneous activity almost disappears, CRFs shrink, and orientation selectivity is high. Keywords: behaving monkey, circular variance, primary visual cortex, receptive fields, spontaneous activity, V1 layers Stimulus selectivity in V1 of alert monkeys 3 Orientation selectivity is of prime importance to the brain's ability to analyze the visual scene. The organization of V1 into orientation columns is evidence of the functional importance of this feature Extensive qualitative evidence shows that cells weakly selective for orientation tend to be spontaneously active, and that the level of spontaneous activity and the level of orientation selectivity covary throughout the orientation column. Many authors agree that in layer 4C, orientation selectivity is weak; this is particularly evident in layer 4C where most cells lack orientation selectivity Contrary to the classical view, Ringach and colleagues Stimulus selectivity in V1 of alert monkeys 4 These results have led to the proposal that V1 should be considered a diverse, but relatively unspecialized, cortical region We have studied orientation selectivity and its relationship to other physiological features in V1 of alert monkeys, where the potential confounding effects of anesthesia are eliminated. Our quantitative data support the concept of great diversity in orientation selectivity of cells in macaque V1, but they also show that the distribution of orientation tuning is nonuniform. Furthermore, the laminar distribution of orientation selectivity is highly differentiated. Our results suggest that both the diversity of orientation tuning and the intricate laminar distribution result from organization into parallel specialized processes that are precisely linked to later cortical areas. Materials and Methods Six adult female monkeys (2 Macaca fascicularis and 4 M. mulatta) were used. Monkeys were trained to fixate on a light-emitting diode (LED) for water reward. Once the monkey learned the task, a head-holding post and a recording well were implanted under deep anesthesia. All procedures complied with National Institutes of Health guidelines and were approved by the Animal Care and Use Committee of the Schepens Eye Research Institute. Nerve-spike and eye-movement recording Fiber electrodes made from quartz-insulated platinum-tungsten alloy Stimulus presentation In the initial experiments, bar and grating stimuli were displayed on a Barco 7351 monitor at a 60-Hz non interlaced frame rate, with a Truevision ATVista video graphics adapter. In recent experiments, stimuli were displayed on Sony 500 PS monitor at a 160-Hz non interlaced frame rate with a Cambridge Research Systems VSG2/3F video board. Bars were optimized for orientation, length, velocity, and color (green or red), 0.9-or 1-log units brighter or darker than the background of 1 cd/m 2 or 5 cd/m 2 . This luminance is in the low photopic range and stimuli are vividly colored. Chromatic stimuli were generated by activation of individual guns of the monitor. Incremental (bright) bars were presented on a neutral gray background; decremental (dark) bars were presented on a background of a single color After the ocular dominance was established, stimuli were viewed binocularly, unless responses during monocular viewing were substantially stronger. The eye position signal was added to the stimulus position signal at the beginning of each video frame (bars) or in the early experiments with gratings, each second frame ("image stabilization", Stimulus selectivity in V1 of alert monkeys 6 that the maximum delay between shifts in eye position and subsequent corrections could be as long as 28 ms for bars and 44 ms for gratings for the 60-Hz frame rate, and 10 ms for both types of stimuli at the 160-Hz frame rate; thus this procedure was not intended to compensate for the fast saccadic eye movements. Saccades were automatically detected using a stability criterion combined with a velocity threshold of 10°/s Using this approach enabled us to accurately map receptive fields Receptive field mapping The width and location of receptive-field activating regions (ARs) were estimated with increment (Inc) and decrement (Dec) bars (2-16 minarc, mean: 7±3 minarc) swept forward and back at 1.5-7°/s across the receptive field in a direction orthogonal to the optimal orientation axis (AR) is used to distinguish regions that respond to direct stimulation from other (covert) zones that may modify the directly evoked response (e.g., side inhibition or facilitation from subthreshold regions). The bar width was adjusted to generate strong responses and was typically smaller than the AR width for both RF mapping and derivation of orientation tuning. To optimize the precision of measurement and minimize possible effects of response latency, we calculated AR widths using the lowest velocity that elicited a strong response in the data set for each cell. Stimulus selectivity in V1 of alert monkeys 7 Using eye position compensation and appropriate eye movement corrections, average peristimulus time histograms (PSTH) of responses were constructed, and a cumulative curve was superimposed ( To distinguish between complex and simple cells, an overlap index (OI) was calculated Modulation ratio The relative modulation (RM, DeValois et al., 1982; Orientation tuning: circular variance and bandwidth Most of our orientation data were collected with drifting bars that changed in orientation in 10-20° angular steps, ranging from 0° to 180° (zero being "horizontal"). In addition, for some very narrowly tuned cells we used finer steps around the preferred orientation. Tuning curves were based on number of spikes generated by each sweep minus the ongoing activity (see below). Several responses were averaged for each datum. The curves were linearly interpolated with a 1° step and smoothed with a Hanning filter with a 7° half-width at half-height Stimulus selectivity in V1 of alert monkeys 9 The ongoing activity was measured in two ways. For comparison with other physiological properties and to establish the distribution across layers the activity was measured during trials with a blank screen, uniformly illuminated at 1 or 5 cd/m 2 ( For many cells the ongoing activity during trials with repeated stimuli was suppressed during the "interstimulus" interval, when the bar was outside the AR , where r k is the firing rate (spikes/s) or the spike count at orientation k , expressed in radians and re-sampled at equally spaced 0.2618 (15°) intervals from the smoothed curve. The CV ranges from 1 for a completely non-oriented (flat) curve to 0 for an exceptionally oriented (zero response at all orientations except the preferred one) curve. Stimulus selectivity in V1 of alert monkeys 10 The tuning curve half-bandwidth (HBW) at 2 / 1 Assignment to layers From our total sample of 339 cells it was possible to assign 170 cells to the various V1 laminae. In the alert preparation it is not possible to use electrolytic lesions as anatomical markers since such lesions are only detectable for a few days. As detailed in Snodderly and Gur (1995; see also Poggio et el., 1977), we used information from several sources to locate the layer of origin: 1) Distance from the cortical surface; 2) physiological properties -an alternating sequence of layers with high ongoing multiunit background activity (4A, 4C and 6), and layers with very low background activity, (2/3, 4B and 5), in addition to the clusters of direction-selective cells found in layer 4B For 46 cells we had cortical depth, physiological, and anatomical data. These cells were assigned confidence level 1. For the rest of the cells we did not have dye marking. For 86 of these cells, the depth and physiological data could lead to only one interpretation -a particular layer; those cells were assigned confidence level 2. For the remaining 38 cells, the assigned layer was based on the most likely interpretation of the depth and physiological evidence; those cells were assigned confidence level 3. An Stimulus selectivity in V1 of alert monkeys 11 important strength of our assignment scheme is that it correctly places recording sites with high spontaneous multiunit activity in anatomical locations with high cytochrome oxidase activity, as shown independently by three different laboratories Statistical analyses Correlations were calculated using the nonparametric Spearman r, since most variables were not normally distributed. Pairwise comparisons utilized the Mann-Whitney U test or the Wilcoxon matched-pairs signed-ranks test. Values reported for individual parameters are medians ± interquartile range unless otherwise stated. Because we make multiple comparisons, we have adopted p < 0.01 as our criterion for statistical significance. Most analyses were done with custom software written in Matlab (MathWorks). Bimodality of the distributions of orientation selectivity and ongoing activity was tested using Hartigan's dip test Results Data collected Orientation half-bandwidth (HBW) and circular variance (CV) were measured with drifting bars for 339 cells with receptive fields at eccentricities of 4.1 ± 1.9°. Within this narrow range, there was no correlation between orientation selectivity and eccentricity. For nearly all cells, we also measured direction selectivity (n = 323) and Inc AR widths (n = 331). In addition, for the majority of the cells, ongoing activity in the light was recorded (n = 297), CRF widths were measured (n = 242 cells), and the overlap index of Inc and Dec ARs was calculated (n = 218). For a subset of our sample (n = 67), Stimulus selectivity in V1 of alert monkeys 12 suppression by bars wider than the CRF was studied, and for another subset (n = 93), relative modulation to drifting gratings was measured. Using gratings and bars to define orientation tuning In studying orientation selectivity, different groups have used either drifting bars Because of the strong surround suppression in alert monkeys, we were careful to use gratings with the most effective spatial frequency and window size; otherwise, the cell would not respond at orientations that were effective for an optimal drifting bar. Three measures of orientation tuning were compared: peak of the orientation tuning curve, CV, and HBW. The respective mean differences (difference ± SD) and correlations between the values obtained for bars and for gratings were: peak 2.2 ± 16.6º, r = 0.95; CV 0.1 ± 0.16, r = 0.81, and HBW 5.6 ± 11.7º, r = 0.85. The Wilcoxon matched-pairs test showed no difference between preferred orientations derived with bars and gratings (p=0.44), while there were very small but statistically significant decreases of CV and HBW for gratings as compared to bars (p<0.01). Except for measures of relative modulation collected with gratings at the optimal orientation, only data collected with drifting bars are presented for the remainder of the paper. Diversity of orientation selectivity Orientation tuning curves based on spike counts and on peak firing rates for representative neurons in each of the cortical layers are displayed in Neurons in V1 of alert monkeys show a wide range of selectivity for orientation Silent cells with no ongoing activity were particularly selective and they formed a large proportion of the narrowly tuned cells. Furthermore, we wish to emphasize that the narrowness of the tuning curves of the selective cells was not due to low responsivity. With near-optimal stimuli, neither the HBW (r = 0.13) nor the CV (r = 0.12) were significantly correlated with the peak firing rate. A previous analysis of orientation tuning in alert monkeys (Vogels and Orban, 1991) used flashed squareStimulus selectivity in V1 of alert monkeys 14 wave gratings of 8-10º in extent. With these large stimuli, the authors reported that cells with a larger orientation bandwidth had stronger responses (r = 0.4). This finding is consistent with an interpretation that the narrowly tuned cells have small fields with strong inhibitory inputs so that their responses would be reduced by the surround inhibition evoked by such a large stimulus (See Discussion). Bandwidth and circular variance HBW and CV were highly correlated in alert animals ( Orientation selectivity and receptive field size There was a clear correlation between measures of receptive field width (Inc and Dec ARs and CRF) and orientation selectivity were also positively correlated with HBW (r = 0.34; p < 0.001) and CV (r = 0.27; p < 0.001). Stimulus selectivity in V1 of alert monkeys 15 Orientation selectivity and ongoing activity If the level of ongoing activity is indicative of the amount of inhibition experienced by the cell The distribution of ongoing activity was nonuniform panel). Many cells (86/297) were completely silent while a considerable number (71/297) were highly active ( 15 spikes/s). It is worth noting that 25/297 cells had ongoing activity 30 spikes/s. Orientation selectivity and overlap index We have recently shown Stimulus selectivity in V1 of alert monkeys 16 Orientation selectivity and relative modulation For cells studied with drifting gratings, the degree of relative modulation (RM) calculated for maximum response or maximum modulation (see Methods) was not correlated with either HBW or CV (all |r| < 0.1). This lack of correlation is expected since we have shown Orientation selectivity and surround suppression To study the relation between surround suppression and orientation selectivity we measured responses of 67 cells to optimally oriented bars of increasing widths. The reduction of maximal response caused by a bar twice the CRF width as a function of HBW and CV, is shown in Relationship of direction selectivity to orientation selectivity There was a bimodal distribution of our current sample into 73 (23%) directional cells (DI 0.5) and 250 (77%) non directional (DI < 0.5) ones The scatter plots in Direction selectivity and ongoing activity Another feature that direction-selective cells share with other tightly oriented cells is low ongoing activity. Cells with DI 0.5 had median ongoing activity of only 0.1 ± 2.6 spikes/s (n=64), whereas the values for the nondirectional group (DI < 0.5) were 4.1 ± 15.5 spikes/s (n=225), a highly significant difference (p<0.0001, Mann-Whitney test). This difference occurred in spite of the fact that ~25% of non-directional cells had 0 ongoing rate. The correlation between ongoing activity and DI was -0.35 (p<0.01, n=289). Direction selectivity and receptive field characteristics Because orientation tuning is narrower for cells with smaller CRFs and smaller ARs, and direction selectivity is correlated with orientation selectivity, it follows that direction-selective cells should have relatively small CRFs and ARs. Indeed, the direction-selective cells (n = 76) had mean ARs that were smaller (15 ± 17') than those of the nondirectional group (26.2 ± 21.9', n = 252; p < 0.0001, Mann-Whitney test). The correlation between DI and mean AR was small, but significant (r= -0.31, p<0.01). Similarly, just as simple cells were slightly more selective for orientation than complex cells, they were also slightly more biased for direction of movement (median DI 0.3 vs. 0.17; p < 0.01 by Mann-Whitney test). Stimulus selectivity in V1 of alert monkeys 18 Conjunctions of physiological properties of V1 cells To illustrate how six of the physiological features discussed above co-vary, a combined measure of orientation selectivity was created by taking the mean of the normalized values of the CV and HBW, and sorting all cells in our sample according to this mean. The results of this procedure are displayed in In contrast to the co-variation of features displayed in the upper panel of Laminar distributions of neuronal characteristics Orientation selectivity. Here we show that orientation selectivity is nonuniformly distributed across the cortical layers, with a higher proportion of nonselective cells in the input layers 4A, 4C, and 6. In The procedure for assignments using 3 levels of confidence is described in the Methods. Since the pattern of alternation of those measures for each of the 3 levels of confidence was very similar, all data were combined to compute median values for each layer. For both HBW and CV there is a clear alternation of orientation selectivity with the layer of origin. Almost all cells in layer 4C were non oriented, which is consistent with most earlier findings This drop in spontaneous activity has previously been noticed by Anatomical evidence from several laboratories (Yoshioka et al., 1994; Yabuta and Callaway, 1998; Stimulus selectivity in V1 of alert monkeys 20 Ongoing activity and AR width. Since present data show that orientation selectivity is correlated with ongoing activity and receptive field width, we examined how the laminar distributions of these characteristics compare with the distribution of orientation selectivity Direction selectivity. Previous reports have emphasized that direction-selective cells are mainly found in layers 4B, 4C , and 6, which receive either direct or second-stage inputs dominated by the magnocellular pathway (Snodderly and Gur, 1995, and references therein). However, the laminar distribution shown in It is worth noting that in layers 2/3 and 5, 7 of 8 direction selective cells for which we had analog spike Stimulus selectivity in V1 of alert monkeys 21 data had large spikes (1.4-4 mV) with a dominant negative lobe, and recordings were stable for a long time. It is thus likely that these direction selective cells are large pyramidal cells Discussion Many prior studies (see Introduction) have shown that V1 has a highly differentiated organization of orientation selectivity and spontaneous activity. Layers with low spontaneous activity typically contain cells more sharply tuned for orientation. In contrast, a nearly uniform distribution of these physiological properties in V1 has been reported by As part of our studies of V1 of alert monkeys we have examined the relation between orientation selectivity (HBW and CV) and several physiological characteristics. Our work shows, for the first time, that cells highly selective for orientation have a suite of distinctive properties; they tend to have low ongoing activity, small CRFs, and they dominate the direction-selective cells. The receptive field organization of the selective cells includes strong surround suppression Stimulus selectivity in V1 of alert monkeys 22 This report is the first quantitative description of the laminar and sublaminar distribution of orientation selectivity in V1 of alert monkeys. We have compared the laminar distribution of orientation selectivity with the distribution of CRF widths, ongoing activity and direction selectivity to present a more integrated view of V1 organization than is obvious from previous studies. Our quantitative results are consistent with earlier, qualitative, findings and differ from Ringach et al.'s (2002b). We find that layers that receive direct LGN input (4A, 4C , 4C and 6) have cells that are less tuned for orientation than layers that do not receive direct LGN input (2/3, 4B, 4Cm, and 5). Similar alternating patterns of laminar properties are seen for orientation selectivity, ongoing activity, and CRF width, with a more complex distribution of direction selectivity. As part of this description, we present the first physiological evidence to support the anatomical delineation of a distinct sublamina, 4Cm, located between 4C and 4C . The properties of 4Cm cells show that selectivity emerges almost immediately in striate cortex without requiring many synaptic relays. We have shown Our finding that cells selective for orientation are strongly suppressed by a stimulus extending beyond the CRF may be another indication of the importance of inhibition in shaping properties of V1 neurons There is a lively debate about the mechanisms responsible for the dramatic emergence of orientation selectivity in V1 (see reviews by While the median values of orientation selectivity clearly differ in input vs. output layers, there is also much larger variation in the selectivity of individual cells in the input layers. This may indicate that transformation to more