[Show abstract][Hide abstract] ABSTRACT: Primates actively examine the visual world by rapidly shifting gaze (fixation) over the elements in a scene. Despite this
fact, we typically study vision by presenting stimuli with gaze held constant. To better understand the dynamics of natural
vision, we examined how the onset of visual fixation affects ongoing neuronal activity in the absence of visual stimulation.
We used multiunit activity and current source density measurements to index neuronal firing patterns and underlying synaptic
processes in macaque V1. Initial averaging of neural activity synchronized to the onset of fixation suggested that a brief
period of cortical excitation follows each fixation. Subsequent single-trial analyses revealed that 1) neuronal oscillation
phase transits from random to a highly organized state just after the fixation onset, 2) this phase concentration is accompanied
by increased spectral power in several frequency bands, and 3) visual response amplitude is enhanced at the specific oscillatory
phase associated with fixation. We hypothesize that nonvisual inputs are used by the brain to increase cortical excitability
at fixation onset, thus “priming” the system for new visual inputs generated at fixation. Despite remaining mechanistic questions,
it appears that analysis of fixation-related responses may be useful in studying natural vision.
[Show abstract][Hide abstract] ABSTRACT: We measured the timing, areal distribution, and laminar profile of fast, wavelength-insensitive and slower, wavelength-sensitive
responses in V1 and extrastriate areas, using laminar current-source density analysis in awake macaque monkeys. There were
3 main findings. 1) We confirmed previously reported significant ventral–dorsal stream latency lags at the level of V4 (V4
mean = 38.7 ms vs. middle temporal mean = 26.9 ms) and inferotemporal cortex (IT mean = 43.4 ms vs. dorsal bank of the superior
temporal sulcus mean = 33.9 ms). 2) We found that wavelength-sensitive inputs in areas V1, V4, and IT lagged the wavelength-insensitive
responses by significant margins; this lag increased over successive levels of the system. 3) We found that laminar activation
profiles in V4 and IT were inconsistent with “feedforward” input through the ascending ventral cortical pathway; the likely
alternative input routes include both lateral inputs from the dorsal stream and direct inputs from nonspecific thalamic neurons.
These findings support a “Framing” Model of ventral stream visual processing in which rapidly conducted inputs, mediated by
one or more accessory pathways, modulate the processing of more slowly conducted feedforward inputs.
[Show abstract][Hide abstract] ABSTRACT: Recent anatomical, physiological, and neuroimaging findings indicate multisensory convergence at early, putatively unisensory stages of cortical processing. The objective of this study was to confirm somatosensory-auditory interaction in A1 and to define both its physiological mechanisms and its consequences for auditory information processing. Laminar current source density and multiunit activity sampled during multielectrode penetrations of primary auditory area A1 in awake macaques revealed clear somatosensory-auditory interactions, with a novel mechanism: somatosensory inputs appear to reset the phase of ongoing neuronal oscillations, so that accompanying auditory inputs arrive during an ideal, high-excitability phase, and produce amplified neuronal responses. In contrast, responses to auditory inputs arriving during the opposing low-excitability phase tend to be suppressed. Our findings underscore the instrumental role of neuronal oscillations in cortical operations. The timing and laminar profile of the multisensory interactions in A1 indicate that nonspecific thalamic systems may play a key role in the effect.