The human face is the most studied object category in visual neuroscience. In a quest for markers of face processing, event-related potential (ERP) studies have debated whether two peaks of activity – P1 and N170 – are category-selective. Whilst most studies have used photographs of unaltered images of faces, others have used cropped faces in an attempt to reduce the influence of features surrounding the “face–object” sensu stricto. However, results from studies comparing cropped faces with unaltered objects from other categories are inconsistent with results from studies comparing whole faces and objects. Here, we recorded ERPs elicited by full front views of faces and cars, either unaltered or cropped. We found that cropping artificially enhanced the N170 whereas it did not significantly modulate P1. In a second experiment, we compared faces and butterflies, either unaltered or cropped, matched for size and luminance across conditions, and within a narrow contrast bracket. Results of Experiment 2 replicated the main findings of Experiment 1. We then used face–car morphs in a third experiment to manipulate the perceived face-likeness of stimuli (100% face, 70% face and 30% car, 30% face and 70% car, or 100% car) and the N170 failed to differentiate between faces and cars. Critically, in all three experiments, P1 amplitude was modulated in a face-sensitive fashion independent of cropping or morphing. Therefore, P1 is a reliable event sensitive to face processing as early as 100 ms after picture onset.

Most mental processes consist of a number of processing steps that are executed sequentially. The timing of the individual mental operations can usually only be estimated indirectly, from the pattern of reaction times. In vision, however, many processing steps are associated with the modulation of neuronal activity in early visual areas. Here we exploited this association to elucidate the time course of neuronal activity related to each of the self-paced mental processing steps in complex visual tasks. We trained monkeys to perform two tasks, search-trace and trace-search, which required performing a sequence of two operations: a visual search for a specific color and the mental tracing of a curve. We used multielectrode recording techniques to monitor the representations of multiple visual items in area V1 at the same time and found that the relevant curve as well as the target of visual search evoked enhanced neuronal activity with a timing that depended on the order of operations. This modulation of neuronal activity in early visual areas could allow these areas to (1) act as a cognitive blackboard that permits the exchange of information between successive processing steps of a sequential visual task and to (2) contribute to the orderly progression of task-dependent endogenous attention shifts that are driven by task structure and evolve over hundreds of milliseconds.

The recovery of visual performance and pupil responses were investigated in patients with demyelinating optic neuritis (ON) and multiple sclerosis (MS). The pupil constriction amplitude and the time delay (latency) of the pupil response were measured in 14 patients with a history of unilateral ON in response to either achromatic (luminance) or chromatic (isoluminant) stimulus modulation. Five of these subjects were diagnosed later with MS. In addition, we measured detection thresholds for achromatic stimuli using standard visual field perimetry and chromatic thresholds using a new colour assessment and diagnosis (CAD) test that isolates the use of colour signals. The results show that, despite significant improvements in visual function following the acute phase (as assessed using visual acuity and fields), significant pupil response deficits remain. The findings also demonstrate that accurate measurements of pupil responses and chromatic thresholds can reveal deficits that remain undetected with more conventional techniques. These preliminary findings suggest that the techniques described here can provide useful information about remitting and relapsing demyelinative phases, often observed during MS and ON.

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