Response characteristics of the normal retino-cortical pathways as determined with simultaneous recordings of pattern visual evoked potentials and simple motor reaction times

Multiple sensory illusions are evoked during the course of proton therapy

Visual illusions from astronauts in space have been reported to be associated with the passage of high energy charged particles through visual structures (retina, optic nerve, brain). Similar effects have also been reported by patients under proton and heavy ion therapies. This prompted us to investigate whether protons at the Loma Linda University Proton Therapy and Research Center (PTRC) may also affect other sensory systems beside evoking similar perceptions on the visual system. A retrospective review of proton radiotherapy patient records at PTRC identified 29 sensory reports from 19 patients who spontaneously reported visual, olfactory, auditory and gustatory illusions during treatment. Our results suggest that protons can evoke neuronal responses sufficient to elicit conscious sensory illusion experiences, in four senses (auditory, taste, smell, and visual) analogous to those from normal sensory inputs. The regions of the brain receiving the highest doses corresponded with the anatomical structures associated with each type of illusion. Our findings suggest that more detailed queries about sensory illusions during proton therapy are warranted, possibly integrated with quantitative effect descriptions (such as electroencephalography) and can provide additional physiological basis for understanding the effects of protons on central nervous system tissues, needed for radiation risk assessment in advance of deep space human exploration.

First- and Second-Order Stimuli Reaction Time Measures Are Highly Sensitive to Mild Traumatic Brain Injuries

Mild traumatic brain injury (mTBI) has subtle effects on several brain functions that can be difficult to assess and follow up. We investigated the impact of mTBI on the perception of sine-wave gratings defined by first- and second-order characteristics. Fifteen adults diagnosed with mTBI were assessed at 15 days, 3 months, and 12 months postinjury. Fifteen matched controls followed the same testing schedule. Reaction times (RTs) for flicker detection and motion direction discrimination were measured. Stimulus contrast of first- and second-order patterns was equated to control for visibility, and correct-response RT means, standard deviations (SDs), medians, and interquartile ranges (IQRs) were calculated. The level of symptoms was also evaluated to compare it to RT data. In general in mTBI, RTs were longer, and SDs as well as IQRs larger, than those of controls. In addition, mTBI participants' RTs to first-order stimuli were shorter than those to second-order stimuli, and SDs as well as IQRs larger for first- than for second-order stimuli in the motion condition. All these observations were made over the three sessions. The level of symptoms observed in mTBI was higher than that of control participants, and this difference did also persist up to 1 year after the brain injury, despite an improvement. The combination of RT measures with particular stimulus properties is a highly sensitive method for measuring mTBI-induced visuomotor anomalies and provides a fine probe of the underlying mechanisms when the brain is exposed to mild trauma.

Visual evoked potentials to change in coloration of a moving bar

In our previous study we found that it takes less time to detect coloration change in a moving object compared to coloration change in a stationary one (Kreegipuu etal., 2006). Here, we replicated the experiment, but in addition to reaction times (RTs) we measured visual evoked potentials (VEPs), to see whether this effect of motion is revealed at the cortical level of information processing. We asked our subjects to detect changes in coloration of stationary (0(°)/s) and moving bars (4.4 and 17.6(°)/s). Psychophysical results replicate the findings from the previous study showing decreased RTs to coloration changes with increase of velocity of the color changing stimulus. The effect of velocity on VEPs was opposite to the one found on RTs. Except for component N1, the amplitudes of VEPs elicited by the coloration change of faster moving objects were reduced than those elicited by the coloration change of slower moving or stationary objects. The only significant effect of velocity on latency of peaks was found for P2 in frontal region. The results are discussed in the light of change-to-change interval and the two methods reflecting different processing mechanisms.

Visual evoked potentials and reaction time measurements to motion-reversal luminance- and texture-defined stimuli

PURPOSE

Previous studies have suggested that compared to first-order (FO) motion stimuli, second-order (SO) motion stimuli required more cortical time to be processed. The purpose of this study was: 1- to verify this claim with Visual Evoked Potential (VEP) and eye-hand Reaction Time (RT) measurements and 2- examine if the VEP and RT responses are similarly modulated by the same trigger features of the stimuli.

METHODS

The VEPs and eye-hand RT for motion-reversal luminance- (first-order) and texture-defined (second-order) stimuli were recorded from ten normal human subjects. VEPs and RTs were measured for each motion class at eight different modulation depths (from 3 to 100%).

RESULTS

Our results reveal that for stimuli of low contrast levels, the SO-FO timing differences are approximately 100 ms (RT) or 20 ms (VEP), while for contrasts >or= 15-20% (VEP) or >or= 50% (RT), the SO-FO difference is no longer significant (p < 0.007), suggesting either that the brain can no longer distinguish SO from FO stimuli or that in spite of the added complexity of SO stimuli the brain takes equal time to process both.

CONCLUSION

Interestingly, the above contrast discrepancy in SO-FO resolution threshold suggests that, compared to the VEP, the more psychophysical RT measurement can process and thus distinguish a larger spectrum of motion stimuli, thus further confirming the latter measure of the retinocortical processing time as a valid alternative to the VEP.

Can whole brain nerve conduction velocity be derived from surface-recorded visual evoked potentials?

Reed, Vernon, and Johnson [Reed, T. E., Vernon, P. A., & Johnson, A. M. (2004). Sex difference in brain nerve conduction velocity in normal humans. Neuropsychologia, 42, 1709-1714] reported that "nerve conduction velocity" (NCV) of visual transmission from retina to the primary visual area (V1) is significantly faster in males than females. The authors estimated the NCV by dividing head length (nasion-to-inion distance) by the latency of the well-known P100 component of the visual evoked potential (VEP). Here, we critically examine these metrics and we contend that knowledge of the underlying physiology of neural transmission across the initial stages of the visual processing hierarchy dictates that a number of their assumptions cannot be reasonably upheld. Alternative, and we believe, more parsimonious interpretations of the data are also proposed.