Perception

Top – down perceptual influences can bias (or pre-empt) perception. In natural scenes, the receptive fields of neurons in the inferior temporal visual cortex (IT) shrink to become close to the size of objects. This facilitates the read-out of information from the ventral visual system, because the information is primarily about the object at the fovea. Top – down attentional influences are much less evident in natural scenes than when objects are shown against blank backgrounds, though are still present. It is suggested that the reduced receptive-field size in natural scenes, and the effects of top – down attention contribute to change blindness. The receptive fields of IT neurons in complex scenes, though including the fovea, are frequently asymmetric around the fovea, and it is proposed that this is the solution the IT uses to represent multiple objects and their relative spatial positions in a scene. Networks that implement probabilistic decision-making are described, and it is suggested that, when in perceptual systems they take decisions (or ‘test hypotheses’), they influence lower-level networks to bias visual perception. Finally, it is shown that similar processes extend to systems involved in the processing of emotion-provoking sensory stimuli, in that word-level cognitive states provide top – down biasing that reaches as far down as the orbitofrontal cortex, where, at the first stage of affective representations, olfactory, taste, flavour, and touch processing is biased (or pre-empted) in humans.

It is generally accepted that vision first evolved for the distal control of movement and that perception or ‘representational’ vision emerged much later. Vision-for-action operates in real time and uses egocentric frames of reference and the real metrics of the world. Vision-for-perception can operate over longer time scales and is much more scene-based in its computations. These differences in the timing and metrics of the two systems have been examined in experiments that have looked at the way in which each system deals with visual illusions. Although controversial, the consensus is that actions such as grasping and reaching are often unaffected by high-level pictorial illusions, which by definition affect perception. However, recent experiments have shown that, for actions to escape the effects of such illusions, they must be highly practiced actions, preferably with the right hand, and must be directed in real time at visible targets. This latter finding suggests that some of the critical components of the encapsulated (bottom – up) systems that mediate the visual control of skilled reaching and grasping movements are lateralised to the left hemisphere.

Membrane potentials and spike sequences represent the basic modes of cerebral information processing. Both can be externally modulated in humans by quite specific techniques: transcranial direct current stimulation (tDCS) and repetitive transcranial magnetic stimulation (rTMS). These methods induce reversible circumscribed cortical excitability changes, either excitatory or inhibitory, outlasting stimulation in time. Experimental pharmacological interventions may selectively enhance the duration of the aftereffects. Whereas rTMS induces externally triggered changes in the neuronal spiking pattern and interrupts or excites neuronal firing in a spatially and temporally restricted fashion, tDCS modulates the spontaneous firing rates of neurons by changing resting-membrane potential. The easiest and most common way of evaluating the cortical excitability changes is by applying TMS to the motor cortex, since it allows reproducible quantification through the motor-evoked potential. Threshold determinations at the visual cortex or psychophysical methods usually require repeated and longer measurements and thus more time for each data set. Here, results derived from the use of tDCS in visual perception, including contrast as well as motion detection and visuo – motor coordination and learning, are summarised. It is demonstrated that visual functions can be transiently altered by tDCS, as has been shown for the motor cortex previously. Up- and down-regulation of different cortical areas by tDCS is likely to open a new branch in the field of visual psychophysics.

Single pulses of transcranial magnetic stimulation (sTMS) restricted locally to the primary cortical areas for somatosensory and visual input, unlike the effects of repetitive stimulation, usually fail to elicit projected sensations. We tested the effect of sTMS over anterior frontal cortex in facilitating phosphenes from preceding sTMS over calcarine cortex, which alone was rarely effective in eliciting phosphenes. The combined sTMS elicited complex phosphenes, which changed with the site of frontal sTMS and the interstimulus interval. Our results show that sTMS over anterior frontal cortex also improved reporting of weakly illuminated, flashed four-letter stimuli, which permitted its statistical validation. We propose that the present demonstration of frontal cortical facilitation of visual awareness, when combined with the previous finding of projected paresthesias and sense of movement (Amassian et al, 1991 Brain 114 2505 – 2520), provide evidence of a general frontal opening effect on a thalamic gate. Opening this gate facilitates entry of information from primary cortical receiving areas to thalamus. Thereby, the reciprocal thalamocortical interrelations that subserve conscious awareness of sensory stimuli could be fostered.

One of the stable hypotheses in systems neuroscience is the relationship between attention and the enhancement of visual responses when an animal attends to the stimulus in its receptive field (Goldberg and Wurtz, 1972 Journal of Neurophysiology 35 560 – 574). This was first discovered in the superior colliculus of the monkey: neurons in the superficial layers of the superior colliculus responded more intensely to the onset of a stimulus during blocks of trials in which the monkey had to make a saccade to it than they did during blocks of trials in which the monkey had to continue fixating a central point and not respond to the stimulus. This enhancement has been found in many brain regions, including prefrontal cortex (Boch and Goldberg, 1987 Investigative Ophthalmology 28 Supplement, 124), V4 (Moran and Desimone, 1985 Science 229 782 – 784), and lateral intraparietal area (Colby et al, 1996 Journal of Neurophysiology 76 2841 – 2852; Colby and Goldberg, 1999 Annual Review of Neuroscience 22 319 – 349), and even V1 (Lamme et al, 2000 Vision Research 40 1507 – 1521). In these studies the assumption has been that the monkey attended to the stimulus because the stimulus evoked an enhanced response. In the experiments described here we show that for abruptly appearing stimuli, attention is not related to the initial response evoked by the stimulus, but by the activity present on the salience map in the parietal cortex when the stimulus appears. Attention to the stimulus may subsequently, by a top – down signal, sustain the map, but stimuli can as easily be suppressed by top – down features as they can be enhanced.

Synfire chains have long been suggested as a substrate for perception and information processing in the nervous system. However, embedding activation chains in a densely connected nervous matrix risks spread of signal that will obscure or obliterate the message. We used computer modeling and physiological measurements in rat hippocampus to assess this problem of activity broadening. We simulated a series of neural modules with feedforward propagation and random connectivity within each module and from one module to the next. We found that activity broadened as it propagated from one module to the next. This occurred over a wide array of parameters with greater broadening seen with increasing excitatory – excitatory synaptic strength. Activity broadening correlated positively with propagation velocity. Multi-electrode measurements of activity propagation in disinhibited CA1 slice demonstrated broadening of about 50% over 1 mm. Such broadening is a problem for information transfer that must be dealt with in a fully functioning nervous system.

Perception depends not only on sensory input but also on the state of the brain receiving that input. A classic example is perception of a stable visual world in spite of the saccadic eye movements that shift the images on the retina. A long-standing hypothesis is that the brain compensates for the disruption of visual input by using advance knowledge of the impending saccade, an internally generated corollary discharge. One possible neuronal mechanism for this compensation has been previously identified in parietal and frontal cortex of monkeys, but the origin of the necessary corollary discharge remained unknown. Here, we consider recent experiments that identified a pathway for a corollary discharge for saccades that extends from the superior colliculus in the midbrain to the frontal eye fields in the cerebral cortex with a relay in the medial dorsal nucleus of the thalamus. We first review the nature of the evidence used to identify a corollary discharge signal in the complexity of the primate brain and show its use for guiding a rapid sequence of eye movements. We then consider two experiments that show this same corollary signal may provide the input to the frontal cortex neurons that alters their activity with saccades in ways that could compensate for the displacements in the visual input produced by saccadic eye movements. The first experiment shows that the corollary discharge signal is spatially and temporally appropriate to produce the alterations in the frontal-cortex neurons. The second shows that this signal is necessary for this alteration because inactivation of the corollary reduces the compensation by frontal-cortex neurons. The identification of this relatively simple circuit specifies the organization of a corollary discharge in the primate brain for the first time and provides a specific example upon which consideration of the roles of corollary activity in other systems and for other functions can be evaluated.

Our objective was to determine perisaccadic gamma range oscillations in the EEG during voluntary saccades in humans. We evaluated occipital perisaccadic gamma activity both in the presence and absence of visual input, when the observer was blindfolded. We quantified gamma power in the time periods before, during, and after horizontal saccades. The corresponding EEG was evaluated for individual saccades and the wavelet transformed EEG averaged for each time window, without averaging the EEG first. We found that, in both dark and light, parietal and occipital gamma power increased during the saccade and peaked prior to reaching new fixation. We show that this is not the result of muscle activity and not the result of visual input during saccades. Saccade direction affects the laterality of gamma power over posterior electrodes. Gamma power recorded over the posterior scalp increases during a saccade. The phasic modulation of gamma by saccades in darkness—when occipital activity is decoupled from visual input—provides electrophysiological evidence that voluntary saccades affect ongoing EEG. We suggest that saccade-phasic gamma modulation may contribute to short-term plasticity required to realign the visual space to the intended fixation point of a saccade and provides a mechanism for neuronal assembly formation prior to achieving the intended saccadic goal. The wavelet-transformed perisaccadic EEG could provide an electrophysiological tool applicable in humans for the purpose of fine analysis and potential separation of stages of ‘planning’ and ‘action’.

This paper focuses on the epistemic conditions of visual perception, ie it concentrates on the question of what kind of knowledge is required for us in order to be able to see colours and shapes as spatial properties of things. According to contemporary theories of sensory perception that follow the tradition of George Berkeley, like Alva Noe's so-called enactive approach to perception, this type of visual perception requires a certain kind of implicit practical knowledge, namely implicit sensorimotor knowledge of the way sensory stimulation varies as the perceiver moves. Two objections are presented against this central claim of the enactive approach. First, empirical evidence from psychological research on children's cognitive and motor development suggests that visual content is entirely independent of sensorimotor knowledge. Second, the enactive approach gets involved in the characteristic problems of classical sense – datum theories by introducing the extremely problematic claim that the recognition of appearances is the epistemic starting point for the perception of things and their properties.

I pursue here three related aims. First, I criticise some of the metaphysical claims made on behalf of the so-called ‘enactive’ approach to visual experience. Secondly, I explain why the enactive view of visual experience is hard to square with the evidence in favour of the two-visual-systems model of human vision. Finally, I explore one possible way to develop the ‘pre-emptive perception’ framework and explain why, contrary to first appearances, some of the fundamental discoveries of brain mechanisms, whose function might be to underlie pre-emptive perception, do not really support the enactive approach to visual experience.

How can an action to a target be selected without yet knowing what it is? Pre-emptive perception (PEP) is a framework which orders neuronal mechanisms in association with voluntary actions before an action is started and until it is completed. It is assumed that PEP serves the purpose of perception, but a conscious, perceptual identification of the goal is not obligatorily completed during the time period of PEP itself. The concept of PEP is that the brain pre-emptively optimizes an action plan to maximize eventual perception, even before being sure what the goal is.

Experimental studies of voluntary saccadic eye movements are considered as prototypic activity within the framework of PEP. The core concept of pre-emption is that a particular saccade is selected while a large number of other possible actions are deselected. Pre-emptive computations include mechanisms associated with internal context and reward. Neurophysiological studies which show anatomically and functionally separate cortical and some subcortical neuronal groups in computing saccades are summarized.

There is a potential relationship of PEP as a neurobiological framework and some philosophical concepts. Terms for processes between planning and action, such as intention, anticipation, and attention, are often incongruent in everyday language and in epistemology. It is proposed here that a scrutiny of these terms can be rigorously approached by temporal subdivision of PEP and conversely, clear definitions of these terms can lead to organized experimental designs of cognitive neurobiology. The temporal subdivision of PEP allows a critique of The Will in the definition of Schopenhauer and distinguishes it from the ‘free will’.