Detecting changes is a critical feature of sensory processing. In the visual system, light increments and decrements are processed through parallel ON and OFF channels that converge in the primary visual cortex (V1). Similarly in the auditory system, UP/DOWN signals (stimulus onset/offset) are carried by non-overlapping sets of neurons that converge in the primary auditory cortex (A1) (Scholl et al., 2010). A recent study has shown that responses of V1 neurons are greatly suppressed by auditory signals in rodents (Iurilli et al., 2012). However, it is unclear whether auditory signals may modulate ON and OFF channels differently. Moreover, the modulation of V1 neurons may depend on the degree of congruence between auditory and visual stimuli (e.g. UP-ON versus DOWN-ON). We addressed these questions by recording from V1 neurons in awake Long-Evans rats. Brief auditory (pure tone) and visual (full field) stimuli were presented simultaneously (0.2 s), and a total of 25 bimodal stimulation conditions (5x5) were used (auditory: mean amplitude: 80dB; UP/DOWN: +13% and +25%; visual: mean amplitude: 70cd/m2; ON/OFF: +35% and +70%). Preliminary results showed that the effect of bimodal stimulation was mostly sub-additive – the bimodal responses were weaker than the linear mathematic combination of the two excitatory unimodal responses. Among the four bimodal conditions, neural responses in the OFF-DOWN condition tend to be the most sub-additive. Overall, these results suggest that a linear combination of A1 and V1 responses cannot explain the complex interaction between ON/OFF channels and UP/DOWN channels.

Primary visual cortex; audiovisual integration; rat