The effect of contrast and spatial frequency on the visual evoked potential of the hooded rat

Electronic photoreceptors enable prosthetic visual acuity matching the natural resolution in rats

Localized stimulation of the inner retinal neurons for high-acuity prosthetic vision requires small pixels and minimal crosstalk from the neighboring electrodes. Local return electrodes within each pixel limit the crosstalk, but they over-constrain the electric field, thus precluding the efficient stimulation with subretinal pixels smaller than 55 μm. Here we demonstrate a high-resolution prosthetic vision based on a novel design of a photovoltaic array, where field confinement is achieved dynamically, leveraging the adjustable conductivity of the diodes under forward bias to turn the designated pixels into transient returns. We validated the computational modeling of the field confinement in such an optically-controlled circuit by in-vitro and in-vivo measurements. Most importantly, using this strategy, we demonstrated that the grating acuity with 40 μm pixels matches the pixel pitch, while with 20 μm pixels, it reaches the 28 μm limit of the natural visual resolution in rats. This method enables customized field shaping based on individual retinal thickness and distance from the implant, paving the way to higher acuity of prosthetic vision in atrophic macular degeneration.

On optimal coupling of the 'electronic photoreceptors' into the degenerate retina

Objective

To restore sight in atrophic age-related macular degeneration, the lost photoreceptors can be replaced with electronic implants, which replicate their two major functions: (1) converting light into an electric signal, and (2) transferring visual information to the secondary neurons in the retinal neural network—the bipolar cells (BC). We study the selectivity of BC activation by subretinal implants and dynamics of their response to pulsatile waveforms in order to optimize the electrical stimulation scheme such that retinal signal processing with 'electronic photoreceptors' remains as close to natural as possible.

Approach

A multicompartmental model of a BC was implemented to simulate responses of the voltage-gated calcium channels and subsequent synaptic vesicle release under continuous and pulsatile stimuli. We compared the predicted response under various frequencies, pulse durations, and alternating gratings to the corresponding experimental measurements. In addition, electric field was computed for various electrode configurations in a 3-d finite element model to assess the stimulation selectivity via spatial confinement of the field.

Main results

The modeled BC-mediated retinal responses were, in general, in good agreement with previously published experimental results. Kinetics of the calcium pumps and of the neurotransmitter release in ribbon synapses, which underpin the BC's temporal filtering and rectifying functions, allow mimicking the natural BC response with high frequency pulsatile stimulation, thereby preserving features of the retinal signal processing, such as flicker fusion, adaptation to static stimuli and non-linear summation of subunits in receptive field. Selectivity of the BC stimulation while avoiding direct activation of the downstream neurons (amacrine and ganglion cells—RGCs) is improved with local return electrodes.

Significance

If the retinal neural network is preserved to a large extent in age-related macular degeneration, selective stimulation of BCs with proper spatial and temporal modulation of the extracellular electric field may retain many features of the natural retinal signal processing and hence allow highly functional restoration of sight.

Characteristics of prosthetic vision in rats with subretinal flat and pillar electrode arrays

Objective

Retinal prostheses aim to restore sight by electrically stimulating the surviving retinal neurons. In clinical trials of the current retinal implants, prosthetic visual acuity does not exceed 20/550. However, to provide meaningful restoration of central vision in patients blinded by age-related macular degeneration (AMD), prosthetic acuity should be at least 20/200, necessitating a pixel pitch of about 50 μm or lower. With such small pixels, stimulation thresholds are high due to limited penetration of electric field into tissue. Here, we address this challenge with our latest photovoltaic arrays and evaluate their performance in vivo.

Approach

We fabricated photovoltaic arrays with 55 and 40 μm pixels (a) in flat geometry, and (b) with active electrodes on 10 μm tall pillars. The arrays were implanted subretinally into rats with degenerate retina. Stimulation thresholds and grating acuity were evaluated using measurements of the visually evoked potentials (VEP).

Main results

With 55 μm pixels, we measured grating acuity of 48 ± 11 μm, which matches the linear pixel pitch of the hexagonal array. This geometrically corresponds to a visual acuity of 20/192 in a human eye, matching the threshold of legal blindness in the US (20/200). With pillar electrodes, the irradiance threshold was nearly halved, and duration threshold reduced by more than three-fold, compared to flat pixels. With 40 μm pixels, VEP was too low for reliable measurements of the grating acuity, even with pillar electrodes.

Significance

While being helpful for treating a complete loss of sight, current prosthetic technologies are insufficient for addressing the leading cause of untreatable visual impairment—AMD. Subretinal photovoltaic arrays may provide sufficient visual acuity for restoration of central vision in patients blinded by AMD.

A 3D flexible microelectrode array for subretinal stimulation

OBJECTIVE

Various retinal prostheses have been developed to restore the vision for blind patients, and some of them are already in clinical use. In this paper, we present a three-dimensional (3D) microelectrode array for a subretinal device that can effectively stimulate retinal cells.

APPROACH

To investigate the effect of electrode designs on the electric field distribution, we simulated various electrode shapes and sizes using finite element analysis. Based on the simulation results, the 3D microelectrode array was fabricated and evaluated in in vitro condition.

MAIN RESULTS

Through the simulation, we verified that an electrode design of square frustum was effective to stimulate with high contrast. Also, the 3D flexible and transparent microelectrode array based on silicon and polydimethylsiloxane was fabricated using micro-electro-mechanical system technologies. In in vitro experiments, the subretinally positioned 3D microelectrodes properly evoked spikes in retinal ganglion cells. The mean threshold current was 7.4 µA and the threshold charge density was 33.64 µC·cm^-2 per phase.

SIGNIFICANCE

The results demonstrate the feasibility of the fabricated 3D microelectrodes as the subretinal prosthesis. The developed microelectrode array would be integrated with the stimulation circuitry and implanted in animals for further in vivo experiments.

Optimization of pillar electrodes in subretinal prosthesis for enhanced proximity to target neurons

OBJECTIVE

High-resolution prosthetic vision requires dense stimulating arrays with small electrodes. However, such miniaturization reduces electrode capacitance and penetration of electric field into tissue. We evaluate potential solutions to these problems with subretinal implants based on utilization of pillar electrodes.

APPROACH

To study integration of three-dimensional (3D) implants with retinal tissue, we fabricated arrays with varying pillar diameter, pitch, and height, and implanted beneath the degenerate retina in rats (Royal College of Surgeons, RCS). Tissue integration was evaluated six weeks post-op using histology and whole-mount confocal fluorescence imaging. The electric field generated by various electrode configurations was calculated in COMSOL, and stimulation thresholds assessed using a model of network-mediated retinal response.

MAIN RESULTS

Retinal tissue migrated into the space between pillars with no visible gliosis in 90% of implanted arrays. Pillars with 10 μm height reached the middle of the inner nuclear layer (INL), while 22 μm pillars reached the upper portion of the INL. Electroplated pillars with dome-shaped caps increase the active electrode surface area. Selective deposition of sputtered iridium oxide onto the cap ensures localization of the current injection to the pillar top, obviating the need to insulate the pillar sidewall. According to computational model, pillars having a cathodic return electrode above the INL and active anodic ring electrode at the surface of the implant would enable six times lower stimulation threshold, compared to planar arrays with circumferential return, but suffer from greater cross-talk between the neighboring pixels.

SIGNIFICANCE

3D electrodes in subretinal prostheses help reduce electrode-tissue separation and decrease stimulation thresholds to enable smaller pixels, and thereby improve visual acuity of prosthetic vision.

Electronic approaches to restoration of sight

Retinal prostheses are a promising means for restoring sight to patients blinded by the gradual atrophy of photoreceptors due to retinal degeneration. They are designed to reintroduce information into the visual system by electrically stimulating surviving neurons in the retina. This review outlines the concepts and technologies behind two major approaches to retinal prosthetics: epiretinal and subretinal. We describe how the visual system responds to electrical stimulation. We highlight major differences between direct encoding of the retinal output with epiretinal stimulation, and network-mediated response with subretinal stimulation. We summarize results of pre-clinical evaluation of prosthetic visual functions in- and ex vivo, as well as the outcomes of current clinical trials of various retinal implants. We also briefly review alternative, non-electronic, approaches to restoration of sight to the blind, and conclude by suggesting some perspectives for future advancement in the field.

Neurofeedback Therapy for Enhancing Visual Attention: State-of-the-Art and Challenges

We have witnessed a rapid development of brain-computer interfaces (BCIs) linking the brain to external devices. BCIs can be utilized to treat neurological conditions and even to augment brain functions. BCIs offer a promising treatment for mental disorders, including disorders of attention. Here we review the current state of the art and challenges of attention-based BCIs, with a focus on visual attention. Attention-based BCIs utilize electroencephalograms (EEGs) or other recording techniques to generate neurofeedback, which patients use to improve their attention, a complex cognitive function. Although progress has been made in the studies of neural mechanisms of attention, extraction of attention-related neural signals needed for BCI operations is a difficult problem. To attain good BCI performance, it is important to select the features of neural activity that represent attentional signals. BCI decoding of attention-related activity may be hindered by the presence of different neural signals. Therefore, BCI accuracy can be improved by signal processing algorithms that dissociate signals of interest from irrelevant activities. Notwithstanding recent progress, optimal processing of attentional neural signals remains a fundamental challenge for the development of efficient therapies for disorders of attention.

Photovoltaic restoration of sight with high visual acuity

Patients with retinal degeneration lose sight due to the gradual demise of photoreceptors. Electrical stimulation of surviving retinal neurons provides an alternative route for the delivery of visual information. We demonstrate that subretinal implants with 70-μm-wide photovoltaic pixels provide highly localized stimulation of retinal neurons in rats. The electrical receptive fields recorded in retinal ganglion cells were similar in size to the natural visual receptive fields. Similarly to normal vision, the retinal response to prosthetic stimulation exhibited flicker fusion at high frequencies, adaptation to static images and nonlinear spatial summation. In rats with retinal degeneration, these photovoltaic arrays elicited retinal responses with a spatial resolution of 64 ± 11 μm, corresponding to half of the normal visual acuity in healthy rats. The ease of implantation of these wireless and modular arrays, combined with their high resolution, opens the door to the functional restoration of sight in patients blinded by retinal degeneration.

A few observations on linking VEP responses to the magno- and parvocellular systems by way of contrast-response functions

It has been proposed that magno- and parvocellular contributions to Visually Evoked Potentials (VEPs) can be isolated, or differentiated, by noting the contrast-response relationships of the responses. This suggestion is examined quantitatively by determining the similarity between various sets of VEP data that have been attributed to the magno- and parvocellular systems and previously reported contrast-response functions for different kinds of neurons (magno- and parvocellular neurons and V1, V4, and MT cells) and combinations of the contrast-response functions for these neurons. It is found that other neurons, or combinations of other neurons, typically give better fits to the data than do magno- and parvocellular cells. Thus, to attribute VEP responses to the magno- or parvocellular systems based on contrast-responses properties faces difficulties.

SSVEP-modulation by covert and overt attention: Novel features for BCI in attention neuro-rehabilitation

In this pilot study the effect of attention (covert and overt) on the signal detection and classification of steady-state visual-evoked potential (SSVEP) were investigated. Using the SSVEP-based paradigm, data were acquired from 4 subjects using 3 scalp electroencephalography (EEG) electrodes located on the visual area. Subjects were instructed to perform the attention task in which they attended covertly or overtly to either of the stimuli flickering with different frequencies (6, 7, 8 and 9Hz). We observed a decrease in signal power in covert compared to the overt attention. However, there was a consistent pattern in covert attention causing an increase in the power of the 2(nd) harmonic of the attended frequency. Encouraging results of this preliminary study indicates that it can be adapted and implemented in the brain-computer interface (BCI) system which could potentially be used as a neuro-rehabilitation tool for individuals with attention deficit.

Holographic display system for restoration of sight to the blind

OBJECTIVE

We present a holographic near-the-eye display system enabling optical approaches for sight restoration to the blind, such as photovoltaic retinal prosthesis, optogenetic and other photoactivation techniques. We compare it with conventional liquid crystal displays (LCD) or digital light processing (DLP)-based displays in terms of image quality, field of view, optical efficiency and safety.

APPROACH

We detail the optical configuration of the holographic display system and its characterization using a phase-only spatial light modulator.

MAIN RESULTS

We describe approaches to controlling the zero diffraction order and speckle related issues in holographic display systems and assess the image quality of such systems. We show that holographic techniques offer significant advantages in terms of peak irradiance and power efficiency, and enable designs that are inherently safer than LCD or DLP-based systems. We demonstrate the performance of our holographic display system in the assessment of cortical response to alternating gratings projected onto the retinas of rats.

SIGNIFICANCE

We address the issues associated with the design of high brightness, near-the-eye display systems and propose solutions to the efficiency and safety challenges with an optical design which could be miniaturized and mounted onto goggles.

Degeneration of cortical function in the Royal College of Surgeons rat

The purpose of the current study was to determine the progress of cortical functional degeneration in the Royal College of Surgeons (RCS) rat. Cortical responses were measured with optical imaging of intrinsic signals using gratings of various spatial frequencies. Subsequently, electrophysiological recordings were also taken across cortical layers in response to a pulse of broad-spectrum light. We found significant degeneration in the cortical processing of visual information as early as 4 weeks of age. These results show that degeneration in the cortical response of the RCS rat starts before development has been properly completed.

Contrast responses to bright slits of visual cells in the superior colliculus of the albino rat

Contrast is the most effective stimulus in the visual system. The response of single cells to changes in stimulus contrast has been studied in a large variety of animals and the contrast response function determined. In the rat, studies on responses to contrast have been focused primarily in the geniculocortical pathway and there are relatively few in subcortical structures. We report here for the first time the contrast response function of single units located in the superior colliculus (SC) of the albino rat to several stimulus contrast. Cells in the SC require a relatively high contrast to elicit a reliable response and the dynamic response range is restricted to a short contrast interval.

Impairments of Gestalt perception in the intact hemifield of hemianopic patients are reflected in gamma-band EEG activity

Gamma-band responses (GBRs) are associated with Gestalt perception processes. In the present EEG study, we investigated the effects of perceptual grouping on the visual GBR in the perimetrically intact visual field of patients with homonymous hemianopia and compared them to healthy participants. All observers were presented either random arrays of Gabor elements or arrays with an embedded circular arrangement. For the hemianopic patients, the circle was presented in their intact hemifield only. For controls, the hemifield for the circle presentation was counterbalanced across subjects. The participants were instructed to detect the circle by pressing a corresponding button. A wavelet transform based on Morlet wavelets was employed for the calculation of oscillatory GBRs. The early evoked GBR exhibited a larger amplitude and shorter latency for the healthy group compared to hemianopic patients and was associated with behavioral measures. The late total GBR between 200 and 400ms after stimulus onset was significantly increased for Gestalt-like patterns in healthy participants. This effect was not manifested in patients. The present findings indicate deficits in the early and late visual processing of Gestalt patterns even in the intact hemifield of hemianopic patients compared to healthy participants.

Stimulus intensity affects early sensory processing: Visual contrast modulates evoked gamma-band activity in human EEG

We studied the effect of different contrast levels on the visual evoked gamma-band response (GBR) in order to investigate whether the GBR is modulated in a similar manner as previously reported for visual evoked potentials. Previous studies showed that the GBR can be modulated by individual characteristics (age) and experimental conditions (task difficulty, attention). However, stimulus properties, such as size and spatial frequency, also have a large impact on the GBR, which necessitates identification and control of relevant stimulus properties for optimal experimental setups. Twenty-one healthy participants were investigated during a forced-choice discrimination task. Sinusoidal gratings were presented at three contrast levels with a constant spatial frequency of 5 cycles per degree visual arc (cpd). The present data replicate the results reported for visual evoked potentials and exhibit a contrast dependent modulation of the GBR. Gamma activity is increased for higher contrast levels. These results demonstrate the importance of stimulus contrast for evoked gamma activity. Thus, it appears meaningful to control the contrast of stimuli in experiments investigating the role of gamma activity in perception and information processing.

Preservation of visual cortical function following retinal pigment epithelium transplantation in the RCS rat using optical imaging techniques

The aim of this study was to determine the extent of cortical functional preservation following retinal pigment epithelium (RPE) transplantation in the Royal College of Surgeons (RCS) rat using single-wavelength optical imaging and spectroscopy. The cortical responses to visual stimulation in transplanted rats at 6 months post-transplantation were compared with those from age-matched untreated dystrophic and non-dystrophic rats. Our results show that cortical responses were evoked in non-dystrophic rats to both luminance changes and pattern stimulation, whereas no response was found in untreated dystrophic animals to any of the visual stimuli tested. In contrast, a cortical response was elicited in most of the transplanted rats to luminance changes and in many of those a response was also evoked to pattern stimulation. Although the transplanted rats did not respond to high spatial frequency information we found evidence of preservation in the cortical processing of luminance changes and low spatial frequency stimulation. Anatomical sections of transplanted rat retinas confirmed the capacity of RPE transplantation to rescue photoreceptors. Good correlation was found between photoreceptor survival and the extent of cortical function preservation determined with optical imaging techniques. This study determined the efficacy of RPE transplantation to preserve visual cortical processing and established optical imaging as a powerful technique for its assessment.

Temporal aspects of contrast visual evoked potentials in the pigmented rat: effect of dark rearing

Cortical visual evoked potentials (VEPs) in response to gratings temporally modulated in counterphase were recorded in normal and dark-reared pigmented rats. Temporal modulation was either sinusoidal (0.25-15 Hz, steady state condition) or abrupt (0.5 Hz, transient condition). In normals, the amplitude spectrum of contrast VEPs has two peaks (at about 0.5 and 4 Hz) and a high temporal frequency cut-off of the order of 11 Hz. The VEP phase lags with temporal frequency, showing two different linear slopes for separate frequency ranges (0.25-1 Hz and 1-7 Hz) centred on the peaks of the curve. The different slopes correspond to apparent latencies of 500 and 136 msec, respectively. Dark rearing reduced the cut-off frequency by about 3 Hz and increased apparent latencies by about 42 msec in the low temporal frequency range and 30 msec in the high temporal frequency range. The latency of the first peak of transient VEPs was increased by about 47 msec. Results indicate that the frequency response of rat contrast VEPs is qualitatively similar to that of other mammals (including human), albeit shifted to a lower range of temporal frequencies. Dark rearing significantly alters the VEP temporal characteristics, suggesting that visual experience is necessary for their correct development.

Flash and pattern reversal visual evoked potentials in C57BL/6J and B6CBAF1/J mice

Visual system responses (visual evoked potentials) to flash (FVEP) and pattern reversal (PRVEP) stimuli were recorded in mice. Two strains were used: black C57BL/6J mice and agouti B6CBAF1/J mice (first generation offspring of C57BL/6J females and CBA/J males.) Subjects were sedated with ketamine and xylazine. Flash rate (FVEP) and stimulus spatial frequency and pattern reversal rate (PRVEP) were varied to determine optimum stimulus parameters. Normative FVEP and PRVEP data were collected from mice of both strains after determination of optimum parameters. Five positive and four negative alternating peaks were routinely observed in the FVEP, while three positive and three negative alternating peaks were seen with the PRVEP. Varying the flash rate, the pattern reversal rate, and spatial frequency significantly affected nearly all amplitude and latency measures in the responses. Significant differences between strains were seen on some, but not all, latency and amplitude measures when the stimulus parameters were varied.

Visual thresholds in albino and pigmented rats

Albino rats have recently been reported to have increment thresholds against dim backgrounds that are two log units higher than those of pigmented rats. We, on the other hand, have failed to confirm these differences using electroretinogram b-waves and pupillary light reflexes. This paper reports on experiments using evoked potentials from cortex and colliculus and single-unit recordings from colliculus. We recorded visual-evoked potentials from cortex and superior colliculus in the strains of albino (CD) and pigmented (Long-Evans) rats used in the earlier studies. Thresholds were determined on eight fully dark-adapted animals by extrapolating intensity-response curves to the point at which there was zero evoked potential. The average dark-adapted threshold for the visual-evoked cortical potential was -5.6 log cd/m2 in pigmented and -5.80 log cd/m2 in albino animals. The average dark-adapted threshold for the superior colliculus evoked response was -5.54 log cd/m2 in pigmented and -5.84 log cd/m2 in albinos. The differences were not statistically significant. On the same apparatus, the average absolute threshold for three human observers was -5.3 log cd/m2, a value close to the rat dark-adapted thresholds. Thus, visual-evoked cortical potentials and superior collicular evoked potentials failed to confirm the report of higher dark-adapted thresholds for albinos. In addition, we find that single units in superior colliculus in the albino rat respond to very dim flashes.

Rat and human visual-evoked potentials recorded under comparable conditions: a preliminary analysis to address the issue of predicting human neurotoxic effects from rat data

Pattern-onset visual-evoked potentials (VEPs) were recorded from rats and humans in order to perform cross-species comparison of neuronal functional properties reflected by the early VEP components. The spatial frequency of a sinusoidal test grating was varied in Experiment 1. For both species, amplitude of the first positive VEP component was larger at low spatial frequency and decreased as spatial frequency increased. The immediately succeeding negative component was small at low spatial frequency and was of maximal amplitude at moderate spatial frequency. The effects of stationary pattern adaptation on these components were investigated in Experiment 2. Subjects viewed either a blank field or the test grating prior to recording VEPs. For both species, adaptation had no effect on the positive component but strongly attenuated the negative component. Experiment 3, in which only humans were tested, indicated that the negative component was of cortical origin. Only cortical neurons are known to be orientation selective, and the effect of adaptation diminished as the orientation difference between the adaptation and test gratings increased. These results suggest that the early positive and negative components arise from parallel visual pathways, and that the rat components may reflect visual processes qualitatively similar to those of humans.

Effect of noradrenergic denervation on task-related visual evoked potentials in rats

The present study examines whether destruction of the noradrenergic innervation of the forebrain interferes with the processing of sensory information in a manner that results in impaired selective attention. Electro-cortical responses to task-relevant and irrelevant stimuli were found to be sensitive indicators of the rat's attention to the stimuli. The amplitude of the response to the task-relevant stimulus increased as the rat's performance improved. The response to irrelevant flashes of light depended on the predictability of the flashes and on the rat's level of arousal. Noradrenergic denervation (with the selective neurotoxin DSP4) did not affect either the behavioural response to a visual stimulus which the rat had been trained to respond to for a food reward, or the late positive potential evoked by this stimulus. Neither did it affect the response to continuous (temporally predictable) flashes of light that were irrelevant to the task. Although the response to unpredictable flashes was also largely unaffected, we did find an additional late component in this response after DSP4 treatment. These results show that the noradrenergic innervation of the occipital cortex does not always regulate the extent to which visual stimuli are processed, but that noradrenergic neurotransmission may be activated in order to diminish excessive processing of unexpected stimuli.

Focal lesions of visual cortex--effects on visual evoked potentials in rats

Focal lesions were placed in the visual cortex of Long-Evans hooded rats, immediately below skull screw recording electrodes. Lesions were produced by heat, and extended an average depth of about 0.9 mm below the cortical surface. Evoked potentials recorded from the electrode overlying the cortical lesion were compared with simultaneously recorded potentials from a contralateral homotopic site. The effects of the lesion were selective. Flash-evoked potential peaks P1, P2, and N2 were depressed by the lesion, and peaks N1 and P3 were augmented; peak N3 was unaffected. Pattern reversal evoked potential peak N3 was depressed by the lesion, and peaks N1 and P2 were made more distinct. The results emphasized that different peaks have different generators, and suggest in particular that flash-evoked potential peaks P1 and N2, and peak N3 of the pattern reversal-evoked potential require the superficial layers of the cortex.

Visual resolution and sensitivity in a nocturnal primate (galago) measured with visual evoked potentials

Visual resolution and contrast sensitivity were examined in anesthetized, paralyzed galagos using visual evoked potentials (VEPs) resulting from stimulation with phase-reversed sinewave gratings. Spatial frequency vs contrast response functions were band-pass with peak sensitivity at 0.2-0.4 c/deg and a high frequency cut-off between 1.6 and 3 c/deg. Peak contrast sensitivities (estimated from extrapolation of contrast response functions) varied across animals from 10 to 170. Variation of the stimulus modulation rate showed that best responses occurred at 1 Hz with an upper limit of 6-16 Hz. As in other primates, an oblique effect was seen in 6 of 8 animals. The contrast sensitivity function (CSF) determined from cortical VEPs agrees well with the CSFs of cells in the lateral geniculate nucleus, but peak sensitivity and spatial frequency are slightly lower than found for the behavioral CSF. Overall visual performance resembled closely that of another nocturnal species, the cat.

Normal strobe electroretinograms without pattern electroretinograms in albino rats

Electroretinograms (ERGs) were obtained from pigmented and albino rats to step luminance changes of an unpatterned TV screen. Surround luminance was increased until the ERG became small and focal. In pigmented rats the ERG at on was positive, earlier, and about twice the amplitude of the negative ERG at off. All pigmented rats had pattern ERGs-0.5 cycles/deg in dark agouti rats and an octave less in hooded rats. Implicit peak times were similar to that of the sum of on plus off focal ERGs from the same animals (85 ms). In albino rats off responses were more like on. The resultant sum was consequently small. Both peak times were similar and did not move earlier than 120 ms as surround luminance increased. Pattern ERGs could not be recorded from albinos at any spatial frequency or surround luminance. These pigmented rat ERGs seem to have two major components. One follows luminance linearly; the other is a fully rectified nonlinearity with about one-third the amplitude. The albino rat retina apparently lacks the latter component. These deficiencies may occur in albinos of other species and be associated with their visual system abnormalities.

Contrast sensitivity and visual acuity of the pigmented rat determined electrophysiologically

The contrast sensitivity function of the rat was assessed by investigating the relationship between the amplitude of visually evoked cortical potentials (VECP) and the spatial frequency and contrast of grating stimuli. Pattern reversal VECPs were recorded in Area 17 in the region of representation of the central binocular visual field. Transient responses were obtained with a 1 Hz contrast reversal. The mean contralateral monocular CSF peaked around 0.1 c/deg, with a threshold sensitivity of 20-25, low frequency attenuation and a high frequency cut-off of 1.18 c/deg. The mean binocular CSF showed a cut-off of 1.20 c/deg, which matches several behavioural measurements of visual acuity. The greater binocular sensitivity in the low frequency range (0.04 c/deg) could be tentatively attributed to the greater influence of the population of large ganglion cells that reaches its maximal density in the ipsilateral projection.

Chlordimeform produces contrast-dependent changes in visual evoked potentials of hooded rats

Previous experiments found that acute exposure to the insecticide/acaricide, chlordimeform (CDM), produced large increases in the amplitude of pattern reversal evoked potentials (PREPs) without changing the amplitude of flash evoked potentials (FEPs) in the same rats (W. K. Boyes and R. S. Dyer, Exp. Neurol. 86: 434-447, 1984). Current work investigated the influence of physical characteristics of the evoking stimuli on the action of CDM. Adult male Long-Evans rats with epidural visual cortex electrodes were used. In experiment 1, PREPs were elicited with alternating gratings having equal contrast (99%) and a square wave spatial luminance profile at several spatial frequencies. Rats treated 1 h previously with 40 mg/kg CDM had increased PREP amplitudes at 0.1, 0.2, and 0.4 cycles per degree (cpd), but not at 0.8 cpd. No changes were found after 5 mg/kg CDM. In experiment 2, PREPs were elicited with gratings oriented at 0 degrees (horizontal), 45 degrees, 90 degrees, or 135 degrees. Treatment with 40 mg/kg CDM increased PREP amplitudes and latencies regardless of orientation. In experiment 3, FEPs elicited with strobe flashes spanning four log units of intensity showed a small but significant CDM dose X intensity interaction on P2N2 peak-to-peak amplitude. In experiment 4, PREPs were elicited with alternating gratings having a sinusoidal spatial luminance profile, spatial frequency of 0.2 or 0.8 cpd, and contrast ranging from noise levels to 65%. Rats treated with 40 mg/kg CDM showed increased peak-to-peak amplitudes only at 0.2 cpd and only at contrast values above 10%. The failure of CDM to alter PREPs at 0.8 cpd was attributed to low contrast sensitivity at that spatial frequency. The results demonstrated that the action of CDM on visual evoked potentials was dependent on the amount of contrast in the stimulus pattern, and suggested that CDM alters the encoding of visual contrast.