Absence of oblique effects in macaque V1

We previously reported that V2 neurons of macaque monkeys exhibit a clear oblique effect and this orientation anisotropy emerges in V2 based on both the pattern of the local connections that are established before birth and the prolonged experience-dependent modifications of the neural circuitry responsible for orientation tuning of V2 neurons. However, the oblique effect in V2 may simply reflect a similar orientation anisotropy in V1. Although macaque monkeys perceptually show an oblique effect, the previous electrophysiological studies of V1 neurons in non-human primates reported mixed results largely due to small sample sizes in most studies. Therefore, in this study we examined the orientation tuning of a large number of V1 neurons to determine whether there is an oblique effect in V1 of macaque monkeys. Extracellular recording was made in individual V1 neurons of anesthetized and paralyzed adult monkeys and also in 4- and 8-week-old monkeys. Standard sinusoidal grating stimuli (40-80% contrast drifted at 3.1 Hz) were used to determine the preferred orientation and spatial frequency of each unit. Neurons were classified into vertically, horizontally, or obliquely oriented cells. We examined the response properties in 1369 V1 units for adults and 347 units for infant monkeys with respect to their preferred orientation, the peak firing rate and the sharpness of orientation tuning. Regardless of the age of animals, we did not find any oblique effect in V1. Specifically: 1) The proportion of V1 units that preferred the cardinal orientations was nearly identical to that preferred the oblique orientations. Also the proportion of units was similar for the 4 primary orientations, the vertical, horizontal, oblique-45, and oblique-135 orientations. 2) There was no difference in the average peak firing-rate of V1 neurons that preferred the cardinal orientations and those preferred the oblique orientations. 3) The average orientation bandwidth or orientation bias of V1 units was similar for those units preferring the cardinal orientations and those tuned to the oblique orientations. 4) There was no significant effect of stimulus spatial frequency on the orientation anisotropy with respect to the proportion, firing rate, orientation bandwidth or bias of V1 neurons. These results suggest that unlike in cats or ferrets, the oblique effect is absent in V1 of macaque monkeys. Our findings are consistent with the results of imaging study in owl monkeys (Xu et al, 2006) and in human psychophysical studies (Gros et al, 1998; Westheimer, 2003)