To see or not to see; the ability of the magno- and parvocellular response to manifest itself in the VEP determines its appearance to a pattern reversing and pattern onset stimulus

Modulation of the neuronal response in human primary visual cortex by re-entrant projections during retinal input processing as manifest in the visual evoked potential

Initial deflections in the visual evoked potential (VEP) reflect the neuronal process of extracting features from the retinal input; a process not modulated by re-entrant projections. Later deflections in the VEP reflect the neuronal process of combining features into an object, a process referred to as 'object closure' and modulated by re-entrant projections. Our earlier work indicated that the VEP reflects independent neuronal responses processing temporal - and spatial luminance contrast and that these responses arise from an interaction between forward and re-entrant input. In this earlier work, changing the temporal luminance contrast property of a stimulus altered its spatial luminance contrast property. We recorded the VEP in 12 volunteers viewing image pairs of a windmill, regular dartboard or an RMS dartboard rotated by either Π/4, Π/2, 3Π/4 or Π radians with respect to each other. The windmill and regular dartboard had identical white to black ratio, while the two dartboards identical contrast edges per unit area. Rotation varied temporal luminance contrast of a stimulus without affecting its spatial luminance contrast. N75, P100, N135 and P240 amplitude and latency were compared and a source localisation and temporal frequency analysis performed. P100 amplitude signals a neuronal response processing temporal luminance contrast that is modulated by re-entrant projections with fast axonal conduction velocities. N135 and P240 signal the neuronal response processing spatial luminance contrast and is modulated by re-entrant projections with slow axonal conduction velocities. The dorsal stream is interconnected by fast axonal conduction velocities, the ventral stream by slow axonal conduction velocities.

Spatiotemporal phase slip patterns for visual evoked potentials, covert object naming tasks, and insight moments extracted from 256 channel EEG recordings

Phase slips arise from state transitions of the coordinated activity of cortical neurons which can be extracted from the EEG data. The phase slip rates (PSRs) were studied from the high-density (256 channel) EEG data, sampled at 16.384 kHz, of five adult subjects during covert visual object naming tasks. Artifact-free data from 29 trials were averaged for each subject. The analysis was performed to look for phase slips in the theta (4-7 Hz), alpha (7-12 Hz), beta (12-30 Hz), and low gamma (30-49 Hz) bands. The phase was calculated with the Hilbert transform, then unwrapped and detrended to look for phase slip rates in a 1.0 ms wide stepping window with a step size of 0.06 ms. The spatiotemporal plots of the PSRs were made by using a montage layout of 256 equidistant electrode positions. The spatiotemporal profiles of EEG and PSRs during the stimulus and the first second of the post-stimulus period were examined in detail to study the visual evoked potentials and different stages of visual object recognition in the visual, language, and memory areas. It was found that the activity areas of PSRs were different as compared with EEG activity areas during the stimulus and post-stimulus periods. Different stages of the insight moments during the covert object naming tasks were examined from PSRs and it was found to be about 512 ± 21 ms for the 'Eureka' moment. Overall, these results indicate that information about the cortical phase transitions can be derived from the measured EEG data and can be used in a complementary fashion to study the cognitive behavior of the brain.

Parallel processing in human visual cortex revealed through the influence of their neural responses on the visual evoked potential

The neural response in the human visual system is composed of magno-, parvo- and koniocellular input from the retina. Signal differences from functional imaging between health and individuals with a cognitive weakness are attributed to a dysfunction of a specific retinal input. Yet, anatomical interconnections within the human visual system obscure individual contribution to the neural response in V1. Deflections in the visual evoked potential (VEP) arise from an interaction between electric dipoles, their strength determined by the size of the neural population active during temporal - and spatial luminance contrast processing. To investigate interaction between these neural responses, we recorded the VEP over visual cortex of 14 healthy adults viewing four series of windmill patterns. Within a series, the relative area white in a pattern varied systematically. Between series, the number of sectors across which this area was distributed doubled. These patterns were viewed as pattern alternating and on-/off stimuli. P100/P1 amplitude increased linearly with the relative area white in the pattern, while N135/N1 and P240/P2 amplitude increased with the number of sectors of which the area white was distributed. The decreases P100 amplitude with increasing number of sectors is attributed to an interaction between electric dipoles located in granular and supragranular layers of V1. Differences between the VEP components obtained during a pattern reversing display and following pattern onset are accounted for by the transient and sustained nature of neural responses processing temporal - and spatial luminance contrast and ability of these responses to manifest in the VEP.

Neural response during temporal - and spatial luminance contrast processing and its manifestation in the blood-oxygen-level-dependent-signal in striate and extra-striate cortex

The primate visual system has been the prime site for investigating the relationship between stimulus property, neural response and blood-oxygen-level-dependent (BOLD)-signal; yet this relationship remains ill-understood. Electrophysiological studies have shown that the ability to visualise a neural response is determined by stimulus property and presentation paradigm. The neural response in the human visual cortex consists of a phasic response processing temporal and tonic response processing spatial luminance contrast. We investigated their influence on the BOLD signal from the visual cortex. To do so, we compared BOLD signal amplitude from BA17 and BA18 of 15 human volunteers to visual patterns varying the size of the active neural population and the discharge activity of this population. The BOLD signal amplitude in both areas reflected the discharge activity of the active neural population but not the size of the active neural population. For identical stimuli, BOLD signal amplitude in BA17 exceeded than that of BA18. This indicates that the BOLD signal reflects the tonic neural neuronal response during spatial luminance contrast processing. The difference in BOLD signal amplitude between BA17 and BA18 is accounted for by differences in neurophysiological and cytoarchitectonic differences between the two areas. Our findings offer an understanding of the relationship between stimulus property, neural response and the BOLD signal by considering the cytoarchitectonic, and neurophysiological make-up between different cortical areas and the influence of a phasic and tonic neural response on local deoxyhaemoglobin concentration. Conversely, differences in BOLD signal between brain structures and stimuli provide cues to the influence of different neurophysiological mechanisms on the neural response.

An investigation into the relationship between stimulus property, neural response and its manifestation in the visual evoked potential involving retinal resolution

The visual evoked potential (VEP) has been shown to reflect the size of the neural population activated by a processing mechanism selective to the temporal - and spatial luminance contrast property of a stimulus. We set out to better understand how the factors determining the neural response associated with these mechanisms. To do so we recorded the VEP from 14 healthy volunteers viewing two series of pattern reversing stimuli with identical temporal-and spatial luminance contrast properties. In one series the size of the elements increased towards the edge of the image, in the other it decreased. In the former element size was congruent with receptive field size across eccentricity, in the later it was incongruent. P100 amplitude to the incongruent series exceeded that obtained to the congruent series. Using electric dipoles due the excitatory neural response we accounted for this using dipole cancellation of electric dipoles of opposite polarity originating in supra- and infragranular layers of V1. The phasic neural response in granular lamina of V1 exhibited magnocellular characteristics, the neural response outside of the granular lamina exhibited parvocellular characteristics and was modulated by re-entrant projections. Using electric current density, we identified areas of the dorsal followed by areas of the ventral stream as the source of the re-entrant signal modulating infragranular activity. Our work demonstrates that the VEP does not signal reflect the overall level of a neural response but is the result of an interaction between electric dipoles originating from neural responses in different lamina of V1.

Shades of grey; Assessing the contribution of the magno- and parvocellular systems to neural processing of the retinal input in the human visual system from the influence of neural population size and its discharge activity on the VEP

INTRODUCTION

Retinal input processing in the human visual system involves a phasic and tonic neural response. We investigated the role of the magno- and parvocellular systems by comparing the influence of the active neural population size and its discharge activity on the amplitude and latency of four VEP components.

METHOD

We recorded the scalp electric potential of 20 human volunteers viewing a series of dartboard images presented as a pattern reversing and pattern on-/offset stimulus. These patterns were designed to vary both neural population size coding the temporal- and spatial luminance contrast property and the discharge activity of the population involved in a systematic manner.

RESULTS

When the VEP amplitude reflected the size of the neural population coding the temporal luminance contrast property of the image, the influence of luminance contrast followed the contrast response function of the parvocellular system. When the VEP amplitude reflected the size of the neural population responding to the spatial luminance contrast property the image, the influence of luminance contrast followed the contrast response function of the magnocellular system. The latencies of the VEP components examined exhibited the same behavior across our stimulus series.

CONCLUSIONS

This investigation demonstrates the complex interplay of the magno- and parvocellular systems on the neural response as captured by the VEP. It also demonstrates a linear relationship between stimulus property, neural response, and the VEP and reveals the importance of feedback projections in modulating the ongoing neural response. In doing so, it corroborates the conclusions of our previous study.

Stimuli to differentiate the neural response at successive stages of visual processing using the VEP from human visual cortex

BACKGROUND

Clarifying the enigmatic relationship between stimulus property, neural response and the evoked potential is essential if non-invasive functional imaging is to make a meaningful contribution to the understanding of how maturational or degenerative processes influence brain activity. Visual cortex has proven a favourite target to elucidate this relationship. However, to date most studies involving the visual system have yielded inconsistent results or have been strongly criticised.

NEW METHOD

We developed a set of three visual stimuli, two of which either had the same low- or high spatial frequency characteristic. Adult volunteers viewed these as pattern reversing stimuli while the scalp electric potential was recorded using a 10-10 array of electrodes.

RESULTS

Established processing mechanisms of the primate visual system enabled us to link the amplitude of the N75 and P100 to the size of the neural population processing the temporal luminance contrast, and the amplitude of the N135 and P240 to the size of the neural processing the spatial luminance contrast in our stimuli. Calculating the distribution of current source density enabled us to identify the neural source of each VEP component.

CONCLUSIONS

Demonstrating a direct relationship between the temporal- and spatial luminance contrast properties of our stimuli and the size of the neural population involved provides a better understanding of the nature of the relationship between stimulus property, neural response and the VEP. It also shows that EEG can contribute in a significant manner to the study of the influence of maturational or degenerative processes on brain activity.

To see or not to see; the ability of the magno- and parvocellular response to manifest itself in the VEP determines its appearance to a pattern reversing and pattern onset stimulus

INTRODUCTION

The relationship between stimulus property, brain activity, and the VEP is still a matter of uncertainty.

METHOD

We recorded the VEP of 43 volunteers when viewing a series of dartboard images presented as both a pattern reversing and pattern onset/offset stimulus. Across the dartboard images, the total stimulus area undergoing a luminance contrast change was varied in a graded manner.

RESULTS

We confirmed the presence of two independent neural processing stages. The amplitude of VEP components across our pattern reversing stimuli signaled a phasic neural response based on a temporal luminance contrast selective mechanism. The amplitude of VEP components across the pattern onset stimuli signaled both a phasic and a tonic neural response based on a temporal- and spatial luminance contrast selective mechanism respectively. Oscillation frequencies in the VEP suggested modulation of the phasic neural response by feedback from areas of the dorsal stream, while feedback from areas of the ventral stream modulated the tonic neural response. Each processing stage generated a sink and source phase in the VEP. Source localization indicated that during the sink phase electric current density was highest in V1, while during the source phase electric current density was highest in extra-striate cortex. Our model successfully predicted the appearance of the VEP to our images whether presented as a pattern reversing or a pattern onset/offset stimulus.

CONCLUSIONS

Focussing on the effects of a phasic and tonic response rather than contrast response function on the VEP, enabled us to develop a theory linking stimulus property, neural activity and the VEP.