NEURAL RESPONSES TO POLAR, HYPERBOLIC, AND CARTESIAN GRATINGS IN AREAV4 OF THE MACAQUE MONKEY

1. We studied the responses of 103 neurons in visual area V4 of anesth etized macaque monkeys to two novel classes of visual stimuli, polar a nd hyperbolic sinusoidal gratings. We suspected on both theoretical an d experimental grounds that these stimuli would be useful for characte rizing cells involved in intermediate stages of form analysis. Respons es were compared with those obtained with conventional Cartesian sinus oidal gratings. Five independent, quantitative analyses of neural resp onses were carried out on the entire population of cells. 2. For each cell, responses to the most effective Cartesian, polar, and hyperbolic grating were compared directly. In 18 of 103 cells, the peak response evoked by one stimulus class was significantly different from the pea k response evoked by the remaining two classes. Of the remaining 85 ce lls, 74 had response peaks for the three stimulus classes that were al l within a factor of 2 of one another. 3. An information-theoretic ana lysis of the trial-by-trial responses to each stimulus showed that all but two cells transmitted significant information about the stimulus set as a whole. Comparison of the information transmitted about each s timulus class showed that 23 of 103 cells transmitted a significantly different amount of information about one class than about the remaini ng two classes. Of the remaining 80 cells, 55 had information transmis sion rates for the three stimulus classes that were all within a facto r of 2 of one another. 4. To identify cells that had orderly tuning pr ofiles in the various stimulus spaces, responses to each stimulus clas s were fit with a simple Gaussian model. Tuning curves were successful ly fit to the data from at least one stimulus class in 98 of 103 cells , and such fits were obtained for at least two classes in 87 cells. In dividual neurons showed a wide range of tuning profiles, with response peaks scattered throughout the various stimulus spaces; there were no major differences in the distributions of the widths or positions of tuning curves obtained for the different stimulus classes. 5. Neurons were classified according to their response profiles across the stimul us set with two objective methods, hierarchical cluster analysis and m ultidimensional scaling. These two analyses produced qualitatively sim ilar results. The most distinct group of cells was highly selective fo r hyperbolic gratings. The majority of cells fell into one of two grou ps that were selective for polar gratings: one selective for radial gr atings and one selective for concentric or spiral gratings. There was no group whose primary selectivity was for Cartesian gratings. 6. To d etermine whether cells belonging to identified classes were anatomical ly clustered, we compared the distribution of classified cells across electrode penetrations with the distribution that would be expected if the cells were distributed randomly. Cells with similar response prof iles were often anatomically clustered. 7. A position test was used to determine whether response profiles were sensitive to precise stimulu s placement. A subset of Cartesian and non-Cartesian gratings was pres ented at several positions in and near the receptive field. The test w as run on 13 cells from the present study and 28 cells from an earlier study. All cells showed a significant degree of invariance in their s electivity across changes in stimulus position of up to 0.5 classical receptive field diameters. 8. A length and width test was used to dete rmine whether cells preferring non-Cartesian gratings were selective f or Cartesian grating length or width. Responses to Cartesian gratings shorter or narrower than the classical receptive field were compared w ith those obtained with full-field Cartesian and non-Cartesian grating s in 29 cells. Of the four cells that had shown significant preference s for non-Cartesian gratings in the main test, none showed tuning for Cartesian grating length or width that would account for their non-Car tesian responses. However, tuning for Cartesian gratings length or wid th was demonstrated in five other cells in the sample. 9. The populati on of V4 neurons displayed a clear bias in their responses in favor of polar and hyperbolic stimuli, and some cells were highly selective fo r these stimuli. The Cartesian stimuli alone could not explain the res ponses of most cells to non-Cartesian stimuli. The fact that nearly al l cells conveyed significant information about all three stimulus clas ses, and that most had identifiable tuning curves in multiple classes, suggests that V4 cells are neither simple feature detectors nor simpl e filters within a single restricted stimulus space. Tuning for multip le stimulus classes may reflect a particular visual processing functio n or a general principle such as efficient image encoding.