It is known that perceptual organization modulates the salience of visual symmetry. Reflectional symmetry is more quickly detected when it is a property of a single object than when it is formed by a gap between two objects. Translational symmetry shows the reverse effect, being more quickly detected when it is a gap between objects. We investigated the neural correlates of this interaction. Electroencephalographic data was recorded from 40 participants who were presented with reflected and translated contours in one- or two-object displays. Half of the participants discriminated regularity, half distinguished number of objects. An event-related potential known as the Sustained Posterior Negativity (SPN) distinguished between reflection and translation. A similar ERP distinguished between one and two object presentations, but these waves summed with the SPN, rather than altering it. All stimuli produced desynchronization of 8-13 Hz alpha oscillations over the bilateral parietal cortex. In the Discriminate Regularity group, this effect was right lateralized. The SPN and alpha desynchronization index different stages of visual symmetry discrimination. However, neither component displayed the Regularity × Objecthood interaction that is observed in speeded discrimination tasks, suggesting that integration of visual regularity with objectness is not inevitable. Instead, both attributes may be processed in parallel and independently.

Neural mechanisms of object recognition seem to rely on activity of distributed neural assemblies coordinated by synchronous firing in the gamma-band range (>20 Hz). In the present electroencephalogram (EEG) study, we investigated induced gamma band activity during the naming of line drawings of upright objects and objects rotated in the image plane. Such plane-rotation paradigms elicit view-dependent processing, leading to delays in recognition of disoriented objects. Our behavioral results showed reaction time delays for rotated, as opposed to upright, images. These delays were accompanied by delays in the peak latency of induced gamma band responses (GBRs), in the absence of any effects on other measures of EEG activity. The latency of the induced GBRs has thus, for the first time, been selectively modulated by an experimental manipulation that delayed recognition. This finding indicates that induced GBRs have a genuine role as neural markers of late representational processes during object recognition. In concordance with the view that object recognition is achieved through dynamic learning processes, we propose that induced gamma band activity could be one of the possible cortical markers of such dynamic object coding.

Pavlovian to Instrumental Transfer (PIT) refers to the behavioral phenomenon of increased instrumental responding for a reinforcer when in the presence of Pavlovian conditioned stimuli that were separately paired with that reinforcer. PIT effects may play an important role in substance use disorders, but little is known about the brain mechanisms that underlie these effects in alcohol consumers. We report behavioral and electroencephalographic (EEG) data from a group of social drinkers (n = 31) who performed a PIT task in which they chose between two instrumental responses in pursuit of beer and chocolate reinforcers while their EEG reactivity to beer, chocolate and neutral pictorial cues was recorded. We examined two markers of the motivational salience of the pictures: the P300 and slow wave event-related potentials (ERPs). Results demonstrated a behavioral PIT effect: responding for beer was increased when a beer picture was presented. Analyses of ERP amplitudes demonstrated significantly larger slow potentials evoked by beer cues at various electrode clusters. Contrary to hypotheses, there were no significant correlations between behavioral PIT effects, electrophysiological reactivity to the cues, and individual differences in drinking behaviour. Our findings are the first to demonstrate a PIT effect for beer, accompanied by increased slow potentials in response to beer cues, in social drinkers. The lack of relationship between behavioral and EEG measures, and between these measures and individual differences in drinking behaviour may be attributed to methodological features of the PIT task and to characteristics of our sample.

Object recognition is achieved through neural mechanisms reliant on the activity of distributed coordinated neural assemblies. In the initial steps of this process, an object's features are thought to be coded very rapidly in distinct neural assemblies. These features play different functional roles in the recognition process - while colour facilitates recognition, additional contours and edges delay it. Here, we selectively varied the amount and role of object features in an entry-level categorization paradigm and related them to the electrical activity of the human brain. We found that early synchronizations (approx. 100 ms) increased quantitatively when more image features had to be coded, without reflecting their qualitative contribution to the recognition process. Later activity (approx. 200–400 ms) was modulated by the representational role of object features. These findings demonstrate that although early synchronizations may be sufficient for relatively crude discrimination of objects in visual scenes, they cannot support entry-level categorization. This was subserved by later processes of object model selection, which utilized the representational value of object features such as colour or edges to select the appropriate model and achieve identification.

Previous studies of feature-selective attention have focused on situations in which attention is directed to one of two spatially superimposed stimuli of equal salience. While such overlapping stimuli should maximize stimulus interactions, it is still unknown how bottom-up biases favoring one or the other stimulus influence the efficiency of feature-selective attention. We examined the integration of bottom-up contrast and top-down feature-selection biases on stimulus processing. Two fully overlapping random dot kinematograms (RDKs) of light and dark dots were presented on a gray background of intermediate luminance. On each trial, human participants attended one RDK to detect brief coherent motion targets, while ignoring any events in the unattended RDK. Concurrently, through changes in background luminance, stimulus contrast could be set to five different levels: the stimuli could either be equal, or one of the two stimuli could have twice or four times the contrast of the other stimulus. This manipulation introduced a bottom-up bias toward the stimulus with the higher contrast while keeping the difference between the stimuli constant. Stimulus processing was measured by means of steady-state visual evoked potentials (SSVEPs). SSVEP amplitudes generally increased with higher contrast of the driving stimulus. At earlier levels of processing, attention increased the slope of this linear relation, i.e., attention multiplicatively enhanced SSVEP amplitudes. However, at later levels of processing, attention had an additive effect. These effects of attention can be attributed to the differential integration of gain enhancement and inhibitory stimulus competition at different levels of the visual processing hierarchy.

Cortical oscillatory activity in the gamma-band range (>30Hz) is a fundamental mechanism of neural coding that arises during a range of cognitive processes in both animals and humans. Since the first report on high frequency oscillatory synchrony between V1 neurons belonging to the same orientation column (Gray and Singer, 1989, PNAS, 86, 1698-1702), the role of such oscillations in visual perception has been extensively researched. Visual stimuli elicit an early, evoked gamma-band response and a later, induced (neither time nor phase-locked) response. An abundance of experimental evidence now links both evoked and induced high frequency oscillations to a range of visual stimulus properties. On the basis of early studies into gamma-band oscillations in vision, induced high frequency oscillatory activity has been proposed as a putative cortical mechanism of coherent percept formation and object representation while evoked high frequency activity was related to the processing of image features. Recent studies demonstrate that both evoked and induced gamma-band activity are correlated with the speed and accuracy of visual detection and discrimination. Furthermore, induced gamma-band oscillations in the visual cortex are also correlated with fixational eye movement patterns. These direct relations between gamma-band activity and the efficacy of visual perception strongly suggest that cortical high frequency synchronisations constitute a neural mechanism that subserves processes essential for the organised intake and analysis of visual information.

Detection and identification of objects are the most crucial goals of visual perception. We studied the role of luminance and chromatic information for object processing by comparing performance of familiar, meaningful object contours with those of novel, non-object contours. Comparisons were made between full-color and reduced-color object (or non-object) contours. Full-color stimuli contained both chromatic and luminance information, whereas luminance information was absent in the reduced-color stimuli. All stimuli were made equally salient by fixing them at multiples of discrimination threshold contrast. In a subsequent electroencephalographic experiment observers were asked to classify contours as objects or non-objects. An advantage in accuracy was found for full-color stimuli over the reduced-color stimuli but only if the contours depicted objects as opposed to non-objects. Event-related potentials revealed the neural correlate of this object-specific luminance advantage. The amplitude of the centro-occipital N1 component was modulated by stimulus class with the effect being driven by the presence of luminance information. We conclude that high-level discrimination processes in the cortex start relatively early and exhibit object-selective effects only in the presence of luminance information. This is consistent with the superiority of luminance in subserving object identification processes.

Visual object processing may follow a coarse-to-fine sequence imposed by fast processing of low spatial frequencies (LSF) and slow processing of high spatial frequencies (HSF). Objects can be categorized at varying levels of specificity: the superordinate (e.g. animal), the basic (e.g. dog), or the subordinate (e.g. Border Collie). We tested whether superordinate and more specific categorization depend on different spatial frequency ranges, and whether any such dependencies might be revealed by or influence signals recorded using EEG. We used event-related potentials (ERPs) and time-frequency (TF) analysis to examine the time course of object processing while participants performed either a grammatical gender-classification task (which generally forces basic-level categorization) or a living/non-living judgement (superordinate categorization) on everyday, real-life objects. Objects were filtered to contain only HSF or LSF. We found a greater positivity and greater negativity for HSF than for LSF pictures in the P1 and N1 respectively, but no effects of task on either component. A later, fronto-central negativity (N350) was more negative in the gender-classification task than the superordinate categorization task, which may indicate that this component relates to semantic or syntactic processing. We found no significant effects of task or spatial frequency on evoked or total gamma band responses. Our results demonstrate early differences in processing of HSF and LSF content that were not modulated by categorization task, with later responses reflecting such higher-level cognitive factors.