Vision Research 50 (2010) 279–283

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Vision Research

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Reduction of negative afterimage duration in Parkinson’s disease patients: A possible role for dopaminergic deficiency in the retinal Interplexiform cell layer

Fatemeh Khadjevand a,b,*, Sohrab Shahzadi c, Abdolhossein Abbassian a a School of Cognitive Sciences (SCS), Institute for Research in Fundamental Sciences (IPM), Tehran 19395-5746, Iran b Interdisciplinary Neuroscience Research Program (INRP), Tehran University of Medical Sciences (TUMS), Tehran, Iran c Neuro-functional Research Center, Shohada Hospital, Shaheed Beheshti University of Medical Sciences, Tehran, Iran article info abstract

Article history: Dopaminergic deficiency may affect Parkinson’s disease patients (PD) in the central as well as the periph- Received 27 February 2009 eral tissues. In the , the neuromodulatory role of the dopaminergic Interplexiform cell layer (IP) Received in revised form 16 September 2009 plays a major role in the retinal light adaptation and may account for the duration of the negative after- image. Here we present results showing a significant reduction of negative afterimage duration in PD patients. This supports the hypothesis that the retinal dopaminergic system may be the main cause for Keywords: the long duration of negative afterimage. We suggest that the observed reduction of afterimage duration Parkinson’s disease is due to possible dopaminergic deficiency in patients with Parkinson’s disease. Retina Ó 2009 Elsevier Ltd. All rights reserved. Dopamine Afterimage Retinal modeling

1. Introduction cells are found in the retina of many species including human (Frederick, Rayborn, Laties, Lam, & Hollyfield, 1982) and the Inter- Parkinson’s disease (PD) is characterized by a general dopamine plexiform cells in humans and other primates which are in contact deficiency (Scatton, Jovoy-Agrid, Rouquier, Dubois, & Agid, 1993). with the amacrine and horizontal cells employ dopamine as a neu- Although motor impairment has been the focus of the clinical eval- rotransmitter (Dowling, 1991). Prolonged visual evoked potential uation and therapy of PD patients, non-motor manifestations such latencies and abnormal electroretinographic patterns, both of as cognitive impairment have also been studied (Lang & Lozano, which respond to levodopa therapy, have been demonstrated in 1998) and evidence for abnormalities of early vision in Parkinson’s Parkinson’s disease patients and in primates with experimental disease is widely reported in the literature (Armstrong, 2008; Parkinsonism (Rodnitzky, 1998). It is reasonable therefore to as- Bodis-Wollner, 1990; Bulens, Meerwaldt, Dan der Wildt, & sume that disruption of dopamine cells in the retina plays a major Keemink, 1986; Crevits, 2003; Djamgoz, Hankins, Hirano, & Archer, role in changes of basic visual in PD patients. 1997; Harnois & Di Paolo, 1990; Harris, 1998; Masson, Mestre, & Among well known studies of perceptual changes in PD patients Blin, 1993; McMahon, Knapp, & Dowling, 1989; Riklan, 1972; are reduction in the ability to detect flicker (Riklan, 1972), abnormal- Rodnitzky, 1998; Wink & Harris, 2000). It is suggested that the ob- ities in spatial contrast sensitivity function (Bulens et al., 1986), de- served changes in perception as well as other non-motor manifes- creased dynamic contrast sensitivity (Masson et al., 1993), changes tations of Parkinson’s disease are due to varied patterns of in the receptive field size of the horizontal cells (McMahon et al., degeneration in dopaminergic neural systems. Some anatomical 1989) and visual abnormalities which are similar to the changes studies show that dopamine in Parkinsonian retina is reduced accompany dark adaptation in normal subjects (the parkinsonian and dopaminergic neurons appear distorted (Bodis-Wollner, retina behaves as though inappropriately dark-adapted) (Wink & 1990; Djamgoz et al., 1997; Harnois & Di Paolo, 1990; Harris, Harris, 2000). In general, the involvement of retinal dopamine defi- 1998; Masson et al., 1993; Rodnitzky, 1998; Wink & Harris, 2000). ciency in some of the visual deficits in Parkinson’s disease is strong Other researches have demonstrated a substantial reduction in (Djamgoz et al., 1997). But, although many such abnormalities retinal dopamine content in MPTP-treated monkeys (Ghilardi, including cognitive deficits are reported in the literature, to our Bodis-Wollner, Onofrj, Marx, & Glover, 1988). Dopamine amacrine knowledge no such study has yet described change in the duration of afterimage perception. This is important with respect to the ques- tion of how dopamine deficiency may influence afterimage duration * Corresponding author. Address: School of Cognitive Sciences (SCS), Institute for Research in Fundamental Sciences (IPM), Tehran 19395-5746, Iran. in PD patients. According to Hayhoe, Benimoff, and Hood (1987) E-mail address: [email protected] (F. Khadjevand). there are three distinct processes underlying light adaptation: a fast

0042-6989/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.visres.2009.09.026 280 F. Khadjevand et al. / Vision Research 50 (2010) 279–283 subtractive process, a slower divisive process and a very slow sub- Brainard, 1997) running on a 3 GHz Intel PC processor. Images tractive process operating over several seconds (Hayhoe, Levin, & were displayed on a 1500 flat, CRT, RGB, LG monitor (1024H Kosel, 1992). Among all the other retinal cells involved in early 800V pixel resolution at 75 Hz frame rate, c = 1.93). stages of it is the Interplexiform (IP) circuit which operates very slowly by neural standards with a time constant on the 2.3. Procedures order of several seconds (Wilson, 1997). One consequence of retinal light adaptation is the production of retinal afterimages which occur In the present study we measured the negative afterimage. when adjacent patches of retina are adapted to different light inten- The experiment was composed of two parts; the training phase sities. In fact, experimental measurements have revealed that nega- and the test phase. Subjects were examined individually in a tive afterimage decays exponentially with a time constant of 4–8 s dark, quiet room while seated in an adjustable chair in front of (Burbeck & Kelly, 1984). a computer. The distance between monitor and subject was This predicts an afterimage decay time well above 7–8 s. 53 cm. According to one model (Wilson, 1997), the slow time course of afterimage decay can be explained by the very slow dynamics of 2.3.1. Training phase the IP layer. In other words the slow build-up and decay of retinal To make sure that the subjects were able to report the presence dopaminergic neuromodulation of the IP cells is assumed to be of a low contrast image accurately (similar to the expected afterim- responsible for the slow decay of the negative afterimage (Wilson, age in the main experiment), subjects took part in the training 1999). A disruption of this dopaminergic neuromodulation process phase. Subjects who were successful in the training phase took part therefore, should produce a corresponding change in the duration in the test phase as well. This way we made sure that performance of afterimage perception. Our results show that this is indeed the differences in the test phase were not caused by any other cogni- case i.e., a reduction in negative afterimage duration is observed tive disorder in the patients, hindering their sensory-motor in patients with Parkinson’s disease where a possible reduction abilities. of retinal dopamine is expected. During the training phase subjects fixated at a red cross in the middle of a white screen. A gray circle appeared and disappeared 2. Methods at random intervals (Fig. 1) Subjects were instructed to push the key whenever the circle showed up and release it as soon as the 2.1. Subjects circle disappeared.

Four patients with Parkinson’s disease participated in the 2.3.2. Test phase experiments. Patients were selected from the outpatient clinic of In the test phase, a common procedure for observing negative neurosurgery of Shahid Beheshti University of Medical Sciences. afterimages was adopted as follows: Patients’ ages varied between 42 and 65. Initially a white circle on a dark background appeared for about Inclusion criterion was the ability of the patients to learn the 20 s and was replaced with a white screen with a fixation point at tasks during the learning phase properly. the middle, after the adaptation period (Fig. 2). Subjects were A neurologist consultation was obtained, who determined sub- asked to press the key whenever they saw the negative afterimage jects were off drug during the test. All patients were off drug dur- and to release it when no afterimage was seen. ing the learning and the test phase. The trial ended after 60 s and the screen turned black at the end Four self-declared neurologically healthy participants who had of each trial. Inter-trail interval was about 5 s and each subject normal mini mental state examinations served as the control group went through 20 trials. for identifying the afterimage duration in the present experimental set-up. 2.4. Data analysis No subject had acute, confounding medical, psychiatric or neu- rologic conditions. To evaluate the difference in afterimage duration between the All subjects had normal or corrected to normal vision. two groups (patients/controls) we used unpaired sample T-test The groups were matched for age. (the significant level was defined as P < 0.05 (two-tailed)). Subject characteristics are listed in Table 1.

3. Results 2.2. Equipments To make sure that the subjects were accurately reporting the The stimulus sequence generation and experimental control appearance of the gray circle during the experiment we calculated were done by MATLAB Psychophysics Toolbox (Pelli, 1997; the error on reporting the duration ‘‘error on reporting the dura- tion” during the training phase by subtracting the onset error from

Table 1 the offset error. Subject characteristics are listed. Subjects with delays or errors on reporting the duration outside an acceptable range of about one second in training phase were ex- Gender Age cluded from the experiment. Patients (n = 4) We excluded the results of two subjects from our final analysis 1M 45 – these subjects showed astonishing low afterimage duration of 2M 42 3F 50about 1–2 s and showed good performance on their training phase, 4M 65therefore they were not excluded during the test phase. The rea- Controls (n = 4) sons are explained in the discussion. 1F 63Four PD patients were chosen for further analysis. 2M 35There was no significant difference of the ‘‘error on reporting 3M 50the duration” of the stimuli in the first part between two groups 4M 56 (patients/controls) (Table 2). F. Khadjevand et al. / Vision Research 50 (2010) 279–283 281

Fig. 1. During this phase subjects fixated at a red cross in the middle of a white screen. A gray circle appeared and disappeared at random intervals, while subjects pressed and held the key for as long as the image remained on the screen. Subjects were instructed to release the key immediately after the disappearance of the stimulus.

Fig. 2. Initially a white circle on a dark background appeared for about 20 s after which a white screen with a fixation point at the middle was shown. Subjects were to press a key upon the appearance of the negative afterimage and to release it when no afterimage was seen. Inter-trail interval was about 5 s.

The negative afterimage durations were significantly shorter in Table 3 patients with PD as compared with control subjects (P- The negative afterimage durations were significantly shorter in patients with PD as compared with control subjects, P-value = 4.63E03. value = 4.63E03) (Table 3). The same results were obtained after correcting for the error on reporting the duration (P- Duration S1 S2 S3 S4 Mean SEM P-value value = 4.40E03). Patients 4.38 4.63 5.28 5.04 4.83 0.20 4.63E03 The afterimage duration in the control group was longer than Controls 9.14 8.63 10.95 7.89 9.15 0.65

6 s in 91% of trials (the oldest subject (C1) showed the longest S: mean afterimage duration of subjects (either patients or controls), SEM: standard afterimage duration). While for patients it was shorter than 6 s in error mean. 83% of the trials. In other words in most trials, PD patients reported

afterimage durations shorter than 6 s while in the case of control subjects almost the opposite held true. Besides, in none of the Table 2 In the training phase the error on reporting the duration was calculated by subtracting off-drug cases where the patients did well on the training phase the onset from the offset error. There was no significant difference of the ‘‘error on was a negative result observed (i.e., longer duration of afterimage reporting the duration” of the stimuli in the first part between two groups (patients/ duration). controls). Fig. 3 shows the result in more details for both the controls and Subjects Error on reporting Error on reporting Error on reporting the four off-drug patients. the onset of stimuli the offset of stimuli the duration of (s) (s) stimuli (s) 4. Discussion P1 0.55 0.69 0.14 P2 0.63 0.24 0.39 P3 0.51 0.39 0.12 The objective of the present study was to investigate the effects P4 1.52 0.58 0.94 of dopamine deficiency in Parkinson patients on afterimage dura- Mean 0.80 0.47 0.32 tion. Based on Wilson’s model of light adaptation, both the forma- SEM 0.24 0.10 0.23 tion and decay of afterimages in the retina can be explained by the C1 0.55 0.75 0.20 very slow neuromodulatory role of the Interplexiform (IP) layer C2 0.73 0.30 0.43 cells. Our prediction was that any deficiency or reduction of dopa- C3 0.40 0.37 0.03 C4 0.46 0.41 0.05 mine in IP layer will result in a corresponding reduction in the per- ception of afterimage duration. Mean 0.53 0.46 0.08 SEM 0.07 0.10 0.13 Here in our experiment, results of subjects agreed well with the perception of afterimage duration (above 7–8 s) in normal popula- P-value 0.35 0.90 0.39 tion reported in literature (Burbeck & Kelly, 1984; Kelly & Marti- P: patient, C: control, SEM: standard error mean. nez-Vriegas, 1993). The results of the present study show a 282 F. Khadjevand et al. / Vision Research 50 (2010) 279–283

Fig. 3. This graph shows the result in more details for both the controls and patients. Vertical axis represents number of trails and horizontal axis represents afterimage duration in second. Each dot represents number of trials subjects reported corresponding afterimage duration. PD curve shows the results for all of the patients as a group. Control curve shows the same for normal subjects. significant reduction in perceiving afterimage duration in PD sub- dicts a slowing down of the corresponding decay and formation of jects compared to the normal control subjects as big as 2–3 s. Gi- afterimage duration. ven that all the other cells including the retinal ganglion cells Two of the PD subjects showed astonishing low afterimage dura- have time constants in the order of milliseconds, had it been that tion of about 1–2 s and also showed good performance on their train- dopamine based processes were operating on a shorter time con- ing phase. This compiled well with the main hypothesis of the stant in the millisecond range the result would have been different present study but we excluded the results of these subjects in the fi- and no reduction would be observed. nal analysis for two reasons: either the afterimage formation process Our result does not rule out other theories of afterimage per- was different or the subjects were almost depleted of dopamine. This ception such as bleaching of the photoreceptors or the involve- of course would be a subject for further research where a larger scale ment of yet higher cortical circuitries. Wilson’s model, however, study of low to high graded Parkinson patients is possible. argues for a direct link between a perceptual experience, negative In one other patient where the patient was tested in both on- afterimage duration, and a specific neural substrate, neuromodu- drug and off drug conditions the result showed a longer duration latory dopaminergic system in the retina. Based on the known ret- of afterimage when the usual medical treatment of levodopa had inal anatomy (Dowling, 1987) the model consists of two distinct been taken around 1–2 h before the experiments. However, as it neural pathways; a direct pathway from the cones to bipolar to is recently reported that dopaminergic medication may improve retinal output ganglion cells and an indirect or adaptive pathway certain types of perceptual performance and hinder others (Ashley consisting of the amacrine to Interplexiform to horizontal cells. et al., 2007), in this case elongation of afterimage might be caused For our present purpose this amarcine to Interplexiform to hori- by a different perceptual mechanism. Therefore, although the long- zontal cell circuitry acts a neuromodulatory role and uses dopa- er duration of perceived afterimage in the on-drug condition is in mine as its neurotransmitter. The two other major circuitries; accordance with our prediction we did not pool the results of this the so-called subtractive feedback circuitry of the horizontal to subject as yet no rising of dopamine level in the retina is reported cone cells and the divisive feedback circuitry of the amacrine to following L-Dopa therapy. the bipolar cells operate on the millisecond range and according A systematic differentiation of the on/off-drug cases of patients to Wilson’s model could not be the basis for the long duration with PD must await a thorough investigation in a future research. of negative afterimage duration. The interplay of the time con- What is important, however, is that in none of the off-drug cases stants in the adaptive pathway makes it the only candidate for where the patients did well on the training phase did we observe causing such effects in the range of several seconds. Reduction a negative result (i.e., longer duration of afterimage duration). of dopamine in the retina of PD patients might result in an in- crease in the neuromodulatory activity of the IP cells which can shorten the time constant of the horizontal cells. Overall Wilson’s 5. Conclusion model well predicts the negative afterimage formation generated by a stationary grating. The important point is that the negative Our results show a significant reduction of negative afterimage afterimage is a byproduct of retinal neuromodulation by IP cells. duration in off-drug patients with Parkinson’s disease. As we ar- Another important aspect of the model is that in case only the di- gued in the discussion section this result correlates well with a rect pathway is active as when a fast flash of light is shown or as a possible deficiency in the retinal dopaminergic system of the PD result of reduction of dopamine in the IP circuitry the model pre- patients. F. Khadjevand et al. / Vision Research 50 (2010) 279–283 283

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