Neurophysiological Correlates of Colour Induction on White Surfaces

The neurosciences at the Max Planck Institute for Biophysical Chemistry in Göttingen

The Max Planck Institute (MPI) for Biophysical Chemistry (Karl-Friedrich Bonhoeffer Institute) was founded in 1971 in Göttingen. Two of the 11 departments at the institute had a neuroscientific focus. Otto D. Creutzfeldt (1927-1992) and Victor P. Whittaker (1919-2016) were directors of the Neurobiological and Neurochemical Departments, respectively. Creutzfeldt's department researched the structure and function of the cerebral cortex, and Whittaker's department concentrated on the biochemical analysis of synapses and synaptic vesicles. Creutzfeldt and Whittaker were already internationally respected scientists when they were appointed to Göttingen. The next generation of departmental directors, Erwin Neher and Bert Sakmann, were "home-grown" researchers from the institute and, during their time as junior group leaders, they developed the so-called patch clamp technique, with which they were able to measure single ion channels in nerve cells. This technique revolutionized neurophysiology, and Neher and Sakmann were awarded the 1991 Nobel Prize in Physiology or Medicine for their work in this area. Neher was appointed director of the Membrane Biophysics Department in 1983 and, since then, his department has mainly examined the role of Ca²⁺ in the release of neurotransmitters at synapses and in the secretion of catecholamines from chromaffin cells. From 1985, Sakmann was director of the Cell Physiology Department, and his laboratory concentrated on the molecular and physiological characterization of transmitter receptors in postsynaptic membranes. In 1989, he was appointed to the MPI for Medical Research in Heidelberg. Reinhard Jahn became director of the Neurobiology Department in 1997, researching the molecular mechanisms of the release of neurotransmitters from the presynaptic terminals, and he discovered several proteins associated with the synaptic vesicles. With their work, Neher, Sakmann, and Jahn have made the MPI for Biophysical Chemistry one of the world's leading research centers for the transmission of signals at synapses.

Chapter 8 Beyond a relay nucleus: neuroimaging views on the human LGN

The lateral geniculate nucleus (LGN) is the thalamic station in the retinocortical projection and has traditionally been viewed as the gateway for sensory information to enter the cortex. Here, we review recent studies of the human LGN that have investigated the retinotopic organization, physiologic response properties, and modulation of neural activity by selective attention and by visual awareness in a binocular rivalry paradigm. In the retinotopy studies, we found that the contralateral visual field was represented with the lower field in the medial-superior portion and the upper field in the lateral-inferior portion of each LGN. The fovea was represented in posterior and superior portions, with increasing eccentricities represented more anteriorly. Functional MRI responses increased monotonically with stimulus contrast in the LGN and in visual cortical areas. In the LGN, the dynamic response range of the contrast function was larger and contrast gain was lower than in the cortex. In our attention studies, we found that directed attention to a spatial location modulated neural activity in the LGN in several ways: it enhanced neural responses to attended stimuli, attenuated responses to ignored stimuli, and increased baseline activity in the absence of visual stimulation. Furthermore, we showed in a binocular rivalry paradigm that neural activity in the LGN correlated strongly with the subjects' reported percepts. The overall view that emerges from these studies is that the human LGN plays a role in perception and cognition far beyond that of a relay nucleus and, rather, needs to be considered as an early gatekeeper in the control of visual attention and awareness.

The neural substrates of conscious color perception demonstrated using fMRI

It is well established that seeing color activates the ventral occipital cortex, including the fusiform and lingual gyri, but less is known about whether the region directly relates to conscious color perception. We investigated the neural correlates of conscious color perception in the ventral occipital cortex. To vary conscious color perception with the stimuli-remaining constant, we took advantage of the McCollough effect, an illusory color effect that is contingent on the orientation of grating stimuli. Subjects were exposed to a specific combination of chromatic grating patterns for 10 min to induce the McCollough effect. We compared brain activities measured while the subjects viewed achromatic grating stimuli before (PRE) and after the induction of the McCollough effect (POST) using functional magnetic resonance imaging (fMRI). There were two groups: one group was informed that they would perceive illusory color during the session (INFORMED group), whereas the other group was not informed (UNINFORMED group). The successful induction of the McCollough effect was confirmed in all subjects after the fMRI experiment; nevertheless, only approximately half of the UNINFORMED subjects had been aware of the color during the POST session, while the other half had not. The left anterior portion of the color-selective area in the ventral occipital cortex, presumably V4alpha, was significantly active in subjects who had consciously perceived the color during MR scan. This study demonstrates the activity in a subregion of the color-selective area in the ventral occipital cortex directly related to conscious color perception.

Representation of color stimuli in awake macaque primary visual cortex

We investigated the responses of single neurons in primary visual cortex (area V1) of awake monkeys to chromatic stimuli. Chromatic tuning properties, determined for homogeneous color patches presented on a neutral gray background, varied strongly between cells. The continuum of preferred chromaticities and tuning widths indicated a distributed representation of color signals in V1. When stimuli were presented on colored backgrounds, chromatic tuning was different in most neurons, and the changes in tuning were consistent with some degree of sensitivity of the neurons to the chromatic contrast between stimulus and background. Quantitatively, the average response changes matched the magnitudes of color induction effects measured in human subjects under corresponding stimulus conditions.

From elements to perception: local and global processing in visual neurons

Gestalt psychologists in the early part of the century challenged psychophysical notions that perceptual phenomena can be understood from a punctate (atomistic) analysis of the elements present in the stimulus. Their ideas slowed later attempts to explain vision in terms of single-cell recordings from individual neurons. A rapprochement between Gestalt phenomenology and neurophysiology seemed unlikely when the first ECVP was held in Marburg, Germany, in 1978. Since that time, response properties of neurons have been discovered that invite an interpretation of visual phenomena (including illusions) in terms of neuronal processing by long-range interactions, as first proposed by Mach and Hering in the last century. This article traces a personal journey into the early days of neurophysiological vision research to illustrate the progress that has taken place from the first attempts to correlate single-cell responses with visual perceptions. Whereas initially the receptive-field properties of individual classes of cells--e.g., contrast, wavelength, orientation, motion, disparity, and spatial-frequency detectors--were used to account for relatively simple visual phenomena, nowadays complex perceptions are interpreted in terms of long-range interactions, involving many neurons. This change in paradigm from local to global processing was made possible by recent findings, in the cortex, on horizontal interactions and backward propagation (feedback loops) in addition to classical feedforward processing. These mechanisms are exemplified by studies of the tilt effect and tilt aftereffect, direction-specific motion adaptation, illusory contours, filling-in and fading, figure--ground segregation by orientation and motion contrast, and pop-out in dynamic visual-noise patterns. Major questions for future research and a discussion of their epistemological implications conclude the article.

The neurophysiological correlates of colour induction, colour and brightness contrast

Psychophysical experiments suggest that colour contrast and colour induction by surround lights can be explained as brightness contrasts (darkness induction) in the spectral region of the surround colour. It follows from this model that a chromatic surround reduces the gain of receptor-ganglion cell channels if the surround colour is in their excitatory spectral region. Thus, a green-sensitive cell (G+/R- or WS in our nomenclature) would respond less to a blue-green stimulus flashed into its receptive field when the surround (5 degrees/20 degrees inner/outer diameter) is illuminated with blue light. Neurophysiological experiments show that this is indeed the case and that such surround-induced response changes are present already in relay cells of the parvocellular layers of the lateral geniculate nucleus (P-LGN) and their retinal afferents. These surround-induced response changes are in qualitative and quantitative agreement with psychophysical experiments. Since the neuronal signal for white consists of a balanced excitation of the M-cone excited, green-blue-sensitive WS-cells and the L-cone excited, yellow-red-sensitive WL-cells, the findings also explain colour induction on white surfaces as well as coloured shadows: during blue surround illumination, white signals from the WS-cells, and during red surround the white signals from the WL-cells are reduced. The neurophysiological surround effects on P-LGN cells are identical but weaker than those produced by light of the same colour shone into the receptive field centres. They are therefore undistinguishable from direct adaptation of those receptors which feed directly into the receptive field of the respective cells. This suggests that they are caused by scattered light reaching the receptive field from the surround.