Object recognition along the ventral stream: effects of visual noise

Psychophysical studies using critical-band masking have established that human object recognition is most impaired by stimulus noise in a narrow, 1.5 octave band of spatial frequency. This shows that visual recognition relies on this narrow band. Here, we investigate the biological basis of this recognition band. We used fMRI to measure BOLD responses along the ventral stream (V1 [->] V2 [->] V3 [->] V4 [->] ventral temporal cortex, VTC) to band-pass noise and to natural images perturbed by that noise. The response to noise alone widens from 2 octaves in V1 to 5 octaves in VTC, diverging more and more from the behavioural 1.5 octave recognition bandwidth. In contrast, a correlation classifier applied to the BOLD response to noisy images shows that the recognition bandwidth is conserved at 2 octaves. This contrast shows that the recognition band is more like a readout constraint than an early sensory filter. Furthermore, this 2-octave band becomes increasingly tolerant to noise, approaching behavioral tolerance in VTC. Thus, it seems that recognition improves along the ventral stream by maintaining a narrow object channel while increasing noise tolerance.