Chen et al., Neuron, 2014 (doi.org/10.1016/j.neuron.2014.03.023)
Abstract
The traditional view on visual processing emphasizes a hierarchy: local line segments are first linked into global contours, which in turn are assembled into more complex forms. Distinct from this bottom-up viewpoint, here we provide evidence for a theoretical framework whereby objects and their parts are processed almost concurrently in a bidirectional cortico-cortical loop. By simultaneous recordings fromV1 and V4 in awake monkeys, we found that information about global contours in a cluttered background emerged initially in V4, started40 ms later in V1, and continued to develop in parallel in both areas.Detailed analysis of neuronal response properties implicated contour integration to emerge from both bottom-up and reentrant processes. Our results point to an incremental integration mechanism:feedforward assembling accompanied by feedback disambiguating to define and enhance the global contours and to suppress background noise. The consequence is a parallel accumulation of contour information over multiple cortical areas.
Figure 2.
V1 neurons started responding to the noise pattern 44ms after its onset, whereas 58ms for V4 neurons. -> An average delay of 14ms is required for feedforward conduction of signals from V1 to V4. (B and C)
A general decrease of stimulus-driven activity along the visual pathway (mean visual-response d'=2.37 for V1, and d'=1.68 for V4). (E)
V1-C sites : Contour facilitation - as the # of bars increases from 1 to 7, there's a progressive increase in activity (As oppose to V1-B sites).
V4 sites also shows the similar enhance responses with increasing contour length.
Contour facilitation
- evident in 90% of V1 sites, and 85% of V4 sites
Background suppression
- observed in 54% of V1 background sites (V1-B)
PSTH (Peristimulus Time Histogram) Analysis
- Contour responses in V4 emerged immediately at the response onset (59ms contour-response latency vs 58ms visual-response latency)
- V1 contour response: 95ms, V1 background suppression: 112ms
On average, the absolute value of contour-response d' increased with contour length for all types of recording sites.
Summary of Fig 2. Contour grouping initially starts in V4 with a fast feedforward process, which is followed by a countercurrent process that engages V1 neurons on the contour for amplification of the signals as well as those on the background for suppression of the noise. The incremental integration process involves bidirectional interactions between visual areas, and continuity is critical for integration of contour segments in both V1 and V4.
(To be added more later, Mar 2020)