Cathode-ray-tube monitor artefacts in neurophysiology

Journal of Neuroscience Methods 141 (2005) 1–7

Cathode-ray-tube monitor artefacts in neurophysiology Andrew J. Zele, Algis J. Vingrys∗ Department of Optometry and Vision Sciences, The University of Melbourne, Parkville, Vic. 3010, Australia Received 26 February 2004; received in revised form 12 May 2004; accepted 12 May 2004

Abstract We demonstrate that cathode-ray-tube (CRT) monitors commonly used as stimulus generators in visual neuroscience produce signal artefacts. This arises from two factors, one being the finite time needed for the raster scan of the CRT to cross the receptive field being stimulated, and the other being the restraint imposed by the impulse response of the phosphor itself. Together these factors result in smearing or blurring that manifests as high frequency noise, distorting the desired signal applied by the investigator. Our analysis identifies those conditions that promote these artefacts and we describe methods for their minimisation. We suggest that a monitor frame rate ≥100 Hz provides a reasonable trade-off between refresh and the generators of high frequency noise. © 2004 Elsevier B.V. All rights reserved. Keywords: CRT monitor; Duty cycle; Flicker sensitivity; Photometry; Luminance

1. Introduction The widespread adoption of cathode-ray-tubes (CRTs) implies that they can, and do, serve as useful stimulus generators for vision related experiments (e.g. Chander and Chichilnisky, 2001; Chichilnisky and Kalmar, 2003; Douglass and Strausfeld, 1996; Groner et al., 1993; Kreiter and Singer, 1996; Muller et al., 2003; Sperling, 1971a; Travis, 1991). Classic CRT calibration procedures have been described elsewhere and need to be adopted to ensure precise control of luminance and chromatic output (e.g. Metha et al., 1993). In this work we consider an important issue, being the effect that the CRT raster scan and discrete temporal refresh can have in characterizing the receptive field profile of a neuron. These discussions are also directly applicable to behavioural experiments. Most investigators will appreciate that CRT-monitors form images by sequential activation of spatially discrete pixel elements (e.g. 1024 × 768 pixels) in a raster scan across the screen, where the raster scans from left to right, and during each frame, from top to bottom (Sperling, 1971b; Travis, 1991). The time taken for a complete scan of the screen determines the frame rate. This leads to an ∗ Corresponding author. Tel.: +61-3-8344-7006; fax: +61-3-9349-7498. E-mail address: [email protected] (A.J. Vingrys).

0165-0270/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jneumeth.2004.05.005

important consideration for behavioural and visual neuroscientists who use CRTs. In that, the time taken by the CRT to scan the perceptive or receptive field of the preparation under study may impact on this response. Another factor that can contribute to artefact generation is the duty cycle of phosphor activation. The duty cycle can be considered as the ratio of the ON-period to the desired activation. In CRT technology, each phosphor rises rapidly on electrical stimulation and decays exponentially with passage of the stimulating electron beam (Vingrys and King-Smith, 1986; Westheimer, 1993). This means that pixel luminescence occurs over a fraction of the dwell time (∼2 ms) created by the monitor refresh (50 Hz gives 20 ms), which is called the duty cycle. The effect that this limited duty cycle can have on neural activity is not trivial as evident in the refresh-locked artefacts observed in vivo by Keating et al. (2001) and in vitro by Chander and Chichilnisky (2001). The significance of these artefacts can be considered in terms of the window of visibility as proposed by Watson et al. (1986). In its original application, the concept was defined using the well specified behavioural limits of the visual system in terms of the perceptual window of visibility (e.g. Crawford, 1947; de Lange, 1954; Robson, 1966; Smith and Pokorny, 1975). Given that these behavioural limits are below the resolving capacity of many neurons in vivo (Berman et al., 1991; Burns et al., 1992; MacLeod and He, 1993; MacLeod et al., 1992; Viswanathan et al., 2002) or that re-

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ported in vitro by single cell recordings (Berry et al., 1997; Chander and Chichilnisky, 2001; Kraft, 1988; Kreiter and Singer, 1996; Reinagel and Reid, 2000; Smith et al., 2001), this gives rise to a neural window of visibility. It is in terms of these windows that we will consider the impact of CRT refresh and pixel activation.

2. Methods We determine the phosphor luminance profile for a single activation and use this to simulate the multiple activations typical of a visual experiment. The signals from such multiple activations were considered by determining the Fourier power spectra of the stimuli and evaluated in terms of the perceptual and neural windows of visibility. We demonstrate how a CRT can blur the signal across a receptive field. 2.1. Stimuli and apparatus For this analysis, we used a commonly available commercial high-resolution CRT operated under the viewing conditions often adopted in vision testing and research. The CRT was a calibrated Hitachi Accuvue HMD-22471 RGB monitor (P22 phosphor) driven by a high-resolution graphics card (Visual Stimulus Generator, VSG2/3; Cambridge Research Systems) hosted in a PC (Compaq 486DX, maths co-processor). The monitor was driven at a frame rate of 120 Hz (8.33 ms refresh) although higher rates were possible. Even though our measures are equipment specific we believe that our comments are not equipment dependent and can be applied to all similar CRT devices. 2.2. Phosphor luminescence profiles It is important to understand the implications that different field stops used in capturing CRT activations can have in predicting the response of cells with different receptive field extents. This will be developed later but for our application, phosphor activation was measured using two calibrated (Optical and Photometric Technology, Melbourne, Australia) photometers having different field stops, one that could measure the luminance profile of a single triad (Pritchard: 2 arc) and a second that could measure over an extended visual angle (Silicon cell: 1◦ arc) typical of some neural receptive fields. The optics of the Pritchard photospectroradiometer were focused onto a single picture element, isolating the record to one group of phosphors (R + G + B) or pixel triad. In comparison, the entrance pupil (1◦ ) of the photosensor captures light over an extended spatial region. Depending on the application, this latter method of measurement may return an inappropriate representation for pixel activation and decay, as will be shown later. In an attempt to capture limited regions of the screen, we modified the photosensor by interposition of several pin-hole apertures (5–15 arc) to act

as field stops. Unfortunately, the field stop modification returned unreliable signals due to the low light levels and the silicon cell was used in its standard configuration (1◦ ) when making measurements. The Pritchard photospectroradiometer (model 1980B) with photo-multiplier was connected to an ADI bio-amplifier (Advanced Digital Instruments, ML135 PowerLab) and high-speed recorder. The ADI was run with a continuous acquisition rate (100 kHz) to simulate a storage cathode-ray-oscilloscope (CRO). As the Pritchard is unlikely to be readily available to all users, we decided to implement a commercially available CRT light-probe adopting a modification of Sperling’s method (1971a). We believe that the photosensor provides an accurate representation of the many commercial spot meters available to neuroscientists for the calibration of CRT luminance. The photosensor was a commercially available 0.44 cm2 silicon cell (OptiCALTM , Cambridge Research Systems) that we connected to the ADI bio-amplifier and high-speed recorder in its storage CRO mode. It was fastened to the centre of the CRT using a suction cap that also served as an ambient light shade. The silicon photosensor was calibrated against a Xenon flash to ensure that response saturation did not affect outcomes (Brainard et al., 2002). In comparison to many of the past investigators who have reported the temporal luminance profiles of a single phosphor (R or G or B, e.g. Vingrys and King-Smith, 1986; Westheimer, 1993), in our analysis we chose to measure the “white” response (R + G + B) as it is typical of many applications where achromatic stimuli are used. In particular it should reflect better the complex operating interactions between phosphor guns, beams and shadow mask elements (Brainard et al., 2002). We show how these two different temporal luminance profiles can result in different signals due to the raster scan and give power at different frequencies, that can easily be quantified. 2.3. Discrete Fourier transforms Discrete Fourier transforms (4096-point DFT) were calculated using a Microsoft-ExcelTM spreadsheet. Independent analyses using MathWorks-MATLABTM gave the same outcomes and we adopted the Excel spreadsheet for convenience. The activations recorded by the photospectroradiometer were used to model stimulus profiles of the time-modulated probes by simulating different refresh rates. In order to consider the changes in a way meaningful to the visual system, we consider the activations in terms of time-modulated contrasts about a mean luminance. By doing so, we acknowledge that although the photoreceptors modulate in relation to the total quantal flux, the response of many neurons shows contrast dependency (e.g. Enroth-Cugell and Robson, 1984) and for this analysis we adopted the approach proposed by others in quantifying the effects of colour changes (Brainard, 1996). As we will show, this provides a useful method for our analysis.

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file can have in limiting the temporal capabilities of stimuli generated by CRTs.

3. Results 3.1. Phosphor luminance profile

Fig. 1. Phosphor activation for a high-resolution RGB monitor determined from an achromatic triad with a screen luminance of 60 cd·m−2 . Measurements were made with a Pritchard photospectroradiometer (model 1980B) with photo-multiplier tube (solid line) using a 2 arc diameter aperture focused in the plane of a single pixel triad and a 0.44 cm2 aperture (∼1◦ ) spot-photometer (dotted line).

The DFT assumes an analysis based on a continuous repeating time series (Bach and Meigen, 1999). In many visual experiments, this is not the case (e.g. Zele and Vingrys, 2001) and a single stimulation (which we will call an impulse) might be the norm. For such stimuli, we have adopted linear interpolation or decimation (Lyons, 1987) to resample the waveform over a binary scale. This method allows frequencies with an integer number of cycles within the sampling window to be cleanly extracted by DFT without the artefacts caused by truncation (Bach and Meigen, 1999; Lyons, 1987). In addition to the impulse, we also considered Gabor (a sinewave windowed by a Gaussian) and Gaussian time series as these have common applications. 2.4. Measuring phosphor activation and decay The equipment (CRT display and photometers) was allowed to warm-up (45–60 min) before use. Phosphor activation and decay was recorded for a white (1931 CIE x = 0.238, y = 0.319) screen luminance of 60 ± 1.0 cd·m−2 . The Pritchard photospectroradiometer with photomultiplier was focused in the plane of a single pixel triad using a 2 arc diameter aperture. The silicon photosensor captured screen activation over 1◦ from a common region of the CRT. The phosphor decay profile returned from the photospectroradiometer (Fig. 1, solid line) was used as a template to simulate time-modulated stimuli for which the frequency spectra were calculated. The analysis was conducted assuming some commonly used monitor frame rates (50, 75 and 120 Hz) corresponding to 20.0, 13.3 and 8.33 ms activation periods, respectively. Comparison was made between theoretical waveforms (e.g. square, sine, Gabor and Gaussian waveforms of short and long duration) that we believe are commonly used in neuroscientific investigations. This provides a basis for the identification of the artefacts introduced by the CRT raster scan and the duty cycle from the phosphor impulse response. In the final analysis we considered the significance that the monitor frame rate and activation pro-

The normalized luminance profile of the Hitachi CRT white phosphor triad driven at 120 Hz is shown in Fig. 1 for the 2 arc aperture (solid line). This profile (white = red + green + blue) has a half height of 0.63 ms and decays to noise (±5%) within 3.2 ms of its 8.33 ms activation window (∼8% duty cycle). Fig. 1 (dashed line) also shows the normalised phosphor luminance profile acquired with the 1◦ photosensor. This has a slower build up, is broader and decays over a longer time-frame than that returned by a single triad. The two profiles demonstrate that different receptive field sizes will perceive different light durations providing that they have the appropriate temporal attributes to integrate the discrete activations of the raster scan. Consistent with the data reported by Keating et al. (2001) and Brainard et al. (2002), the silicon cell returns a luminance profile with a half height of 1.9 ms and a significant persistence of up to 5–6 ms (∼25% duty cycle). 3.2. Power spectra of waveforms generated using non-discrete stimulators Fig. 2A and B shows the power spectra for long (continuous) and short (impulse) duration waveforms (schematics, right hand-side of the normalized power spectra). Fig. 2A shows the expected luminance profile (vertically adjusted) for three commonly used, 60 Hz stimulus waveforms; a continuous sinewave (lower, solid line), gabor (middle; dashed line) and square wave (upper, dotted line), with their relative power spectra. We chose to plot this data on a linear ordinate to emphasise meaningful signals with amplitudes >1/100 of the normalized maximum (2 log units). As would be expected, the normalised frequency spectra for each waveform (sine, square and gabor) is tightly centred on 60 Hz, with the square wave introducing higher frequency harmonics (180 Hz; panel A, dotted line) due to its temporal profile. Reducing the duration of the waveform widens the power spectra (Fig. 2B). Although the power spectra remain centred on the modulation fundamental (60 Hz), the broadened spectra (Fig. 2B) may limit precision in isolating a neuron’s temporal response. A single cycle ON–OFF square wave (Fig. 2B, dashed line) has the broadest frequency response of the three waveforms considered and it is accompanied by very low energy harmonics. The signals shown in Fig. 2 cannot be realized on a CRT due to the discrete phosphor refresh (signalling) returned in Fig. 1. The CRT will act to change the duty cycle of the waveform, and it follows, that the duty cycle will vary ac-

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Fig. 3. Effect of duty cycle on the signal power spectra. Normalised power for a 60 Hz square wave with either a 50% duty cycles (dashed line), 24% (solid line) or 12% duty cycle (dotted line). The fundamental of the waveform is centred on 60 Hz with frame rate artefacts introduced at 120 Hz, as are higher order harmonics (180 Hz). The duty cycles approximate those of the signals shown in Fig. 1. The power spectra for the 12% and 24% duty cycles were shifted horizontally for clarity, by ±5 Hz, respectively.

3.3. Power spectra of waveforms generated using discrete stimulators: CRTs Fig. 2. Power spectra for conventional waveforms (long and short duration) used in neuroscience. (A) Normalised magnitude of a discrete Fourier transform (power spectra) for sinusoidal, gabor and square wave (60 Hz) continuous waveforms (vertically adjusted for clarity) as indicated by the schematic time-modulated profiles given to the right of the figure. (B) The power spectra for a single cycle (impulse) of the sinusoidal, gabor and square waveforms (vertically adjusted) and demonstration of how the reduced temporal duration broadens the frequency spectrum of the stimulus.

cording to the size of the receptive field (or the entrance pupil of the acquisition device; see Fig. 1). Fig. 3 demonstrates the effect that altering the duty cycle (12 and 24%, approximately the duty cycles of the signals shown in Fig. 1) of a 60 Hz continuous square wave (Fig. 3, dashed grey line) can have on the power spectra. Two effects can be observed as the duty cycle decreases. Significant power is introduced at the frame rate (e.g. 120 Hz) and at higher frequencies (e.g. harmonic at 180 Hz), both of which accompany reductions in the energy at the fundamental frequency (60 Hz). Calculation of the signal-to-artefact ratio (i.e. 60:120 Hz and 60:180 Hz) gives an estimate of the energy found at the harmonics (frame rate and higher order) relative to the fundamental. For high duty cycles (upper dashed line), the power at the frame rate is 2.9× the fundamental, and 3.8× at the harmonic. The power found at the higher frequencies (beyond the fundamental frequency) increases at lower duty cycles (middle solid line), having 3.8× the power at the frame rate and 7.5× at the harmonics. Next we show, by analysis, the power spectra of a flickering waveform generated on CRTs having different frame rates.

The stimulus window for a single cycle (∼5 Hz) bipolar probe generated using 50, 75 and 120 Hz frame rates are shown in the left panels and, their respective power spectra of the contrast modulation, in the right panels of Fig. 4. As frame rate increases, the effective duty cycle increases and this modifies the power spectrum of the time-modulated probe (see Fig. 3). At each frame rate, the fundamental corresponds to the modulation frequency of 5 Hz (Fig. 4, right panels). The power spectra, however, contain significant peaks at both the frame rate (arrow) and higher harmonics due to the finite stimulus window and the discrete phosphor sampling. If these 5 Hz stimuli were shown to a neuron having a larger receptive field (as in Fig. 1), the effect would be to alter the ratio of the powers found at the fundamental and frame rate (as per Fig. 3). To consider the effect that these CRT artefacts can have on the visual system, the data from Fig. 4B (50 Hz refresh, 5 Hz pulse) have been redrawn in Fig. 5 along with the human perceptual temporal response (de Lange, 1954) and the horizontal cell (H1) temporal response which represents the output of neurons early in the visual process (Smith et al., 2001). This defines two windows of visibility as detailed in the introduction. It can be seen that the visual system has differential sensitivity to energy at high frequencies, which may influence outcomes. In particular, Fig. 5 demonstrates how the harmonics introduced by the frame rate will become detectable by neurons as frame rates decease. This effect is more pronounced for impulses than for multiple-cycle probes due to the temporal stimulus widow (see Fig. 2). What is most compelling, is that Fig. 5 identifies a range of temporal frequencies (60–80 Hz) where the artefact is perceptually invisible but can elicit neural responses.

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Fig. 4. Stimuli (left) and normalised Fourier magnitudes (right) of a single cycle (∼5 Hz) of flicker generated using three commonly implemented monitor frame rates: 50 Hz (A and B), 75 Hz (C and D) and 120 Hz (E and F). Note how substantial noise is created by the frame rate (arrow).

4. Discussion 4.1. Phosphor activation profile The rise and decay seen in our phosphor triad (Fig. 1) is consistent with previous estimates derived from single phosphor activations (Farrell, 1991; Sperling, 1971b; Vingrys and King-Smith, 1986; Westheimer, 1993). The fast decay means that there is no chance of luminous carry over or phosphor persistence even with a frame rate of up to 200 Hz (5 ms). However, this must be tempered by the operating luminance of the screen: had we used a brighter background, phosphor persistence would have increased. So it is imperative that each experimenter quantify the time course of phosphor activation under the operating conditions common to their laboratory and in cases where phosphor persistence is found, a number of methods are available to

describe and correct for these artefacts (e.g. Metha et al., 1993; Vingrys and King-Smith, 1986; Westheimer, 1993). We have shown how the receptive field of a neuron can modify the intended temporal profile of a CRT phosphor due to its duty cycle or raster scan (Fig. 1). The nature of the phosphor signal is dependent on the screen area over which the signal has been captured and an accurate description of a triad can only be obtained from a region of the screen encompassing a single red, blue and green phosphor (Fig. 1, solid line). Otherwise, the waveform returned with the larger receptive field will represent the output of the three phosphors integrated over the aperture of the neuron during the time needed to complete a scan. The integration of the CRT signal over an extended region causes smearing of the response over space and time (Fig. 1, dotted line). Depending on the size of the receptive field, this may return an inaccurate representation of the timing of the underly-

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Fig. 5. The discrete Fourier transform for a 5 Hz flickering waveform generated on a CRT with a 50 Hz refresh rate redrawn from Fig. 3 to emphasise the scale ranging between 50 and 120 Hz. The psychophysical temporal sensitivity function of de Lange (1954) and that of a horizontal cell (H1) from Smith et al. (2001) have been plotted by the black and grey solid lines, respectively. Note how artefacts introduced by the discrete sampling of the CRT phosphors produce low amplitude signals (60–100 Hz) that are invisible to the observer but can be detected by the neurone.

ing activation (Fig. 1, solid line) as observed by Chander and Chichilnisky (2001). Indeed, broad temporal luminance profiles have been reported in the literature (e.g. see Fig 7 of Brainard et al., 2002; Fig 3 of Keating et al., 2001) and imply that an extended field stop may have been used when defining the luminance profile in these studies. Not surprisingly, the nature of phosphor persistence has been the subject of significant debate (Di Lollo et al., 1994; Irwin, 1994; Westheimer, 1993, 1994). We suggest that, apart from the physical issues constraining phosphor decay (see Groner et al., 1993), the significance of the physiological manifestations of persistence must be evaluated in terms of a mechanism’s receptive field or entrance pupil and we consider this in the next section. As a corollary, we propose that a spot-photometer should be used as a luminance meter and only applied to assay phosphor profiles when its entrance pupil matches that of the detecting mechanism. 4.2. Detecting cathode ray tube signals As we have alluded before, the implications of these measures need to be considered in terms of the window of visibility (Watson et al., 1986) as the issue rests, not with phosphor activation alone, but with the nature of receptive field of the detecting neurone/neurones (Gawne and Woods, 2003). We have shown that these artefacts arise from the discrete phosphor resampling (see Figs. 3 and 4) that occurs within the stimulus window (Watson, 1986). If the preparation under study were to involve a cone dominated process then the 2 aperture is most appropriate and the luminance profile given by the solid line of Fig. 1 is accurate for this purpose. However, preparations with larger receptive fields, such as the rod pathway (Lennie and Fairchild, 1984), will be better represented by the 1◦ aperture. Indeed, the ability of the eye to respond to high temporal frequencies appears to be

an attribute of the local retina as the multi-focal ERG shows a response at monitor refresh (Keating et al., 2001). It is hard to judge the significance of these artefacts because human perception has little sensitivity at high temporal frequencies (Fig. 5, solid black line). We propose that detection mechanisms at the fundamental will be substantially more sensitive than mechanisms located at any higher frequencies, as evident from Fig. 5. So these artefacts are unlikely to influence threshold-related experiments involving low temporal frequencies. Keating et al. (2001) have noted that they might be significant for non-linear processes, however, our analysis did not consider this possibility. It might be expected that these high frequency artefacts will become more significant with stimuli that are presented above threshold, as is the case with an ERG obtained at high luminous energies. Consistent with these predictions, Keating et al. (2001) isolated robust oscillatory artefacts in their 75 Hz CRT-generated multi-focal ERG signals that are not present when obtained using the same stimulus presented on a continuous display (liquid crystal display). Fig. 5 also demonstrates the importance of adopting high frame rates (>100 Hz) as these will have the effect of placing any artefacts at frequencies well beyond the window of visibility of retinal elements. The frequency spectrum of the discrete sampling used by CRTs is dependent on monitor frame rate (Fig. 4) with our analyses demonstrating that the resultant power spectrum is modified by the time between subsequent activations (duty cycle). As the number of discrete activations within a stimulus window increases at higher frame rates, these will act to reduce the number of high frequency transients that may be detected at suprathreshold levels. The implementation of a monitor having a high frame rate will not only decrease the likelihood of artefact detection, but will also provide the user with a larger selection of integer options at high temporal frequencies (see Section 4.3). 4.3. Restrictions imposed by monitor sampling The discrete nature of the video frame rate coupled with the fact that a repetitive waveform comprises two components (ON and OFF) in its period, means that not all integer frequencies can be achieved with a CRT. As a consequence, the potential frequencies form a binary sequence given in Hz = F/2n where, F is the frame rate, and n, is the number of cycles (ON–OFF) presented in the stimulus. For example, a frame rate, F, of 100 Hz and a period comprising 1, 2, 3, 4, . . . , n cycles yields 50, 25, 12.5, 6.25, . . . , x Hz. Note that significant under sampling is found in the high temporal frequency domain and this will become most marked with lower frame rates. Our analyses suggest that a monitor refresh ≥100 Hz provides a good trade-off between sampling (refresh) and the visual response to act as a stimulus generator. Depending on the particular application, the effects of phosphor persistence and frame rate may still prove bothersome so that they

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must be eliminated. In these cases implementation strategies (see previous) or alternative technologies, such as light emitting diodes, liquid crystal display screens or digital light projectors, which do not refresh line-by-line, may need to be considered. However, these latter technologies all have their own particular limitations (see Brainard et al., 2002; Keating et al., 2001; Packer et al., 2001) that need to be fully evaluated before implementation.

Acknowledgements Dr George Smith and Dr Steven Jenkins provided guidance with phosphor luminance measurements. This work is supported by an Australian Research Council Linkage Project (LP0211474) grant.

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