Children with attention deficit hyperactivity disorder (ADHD) experience difficulties because of the inattention, impulsivity and deficit of activity control that can have a severe effect on performance in school, home and community environment.¹ Recently, several studies support the idea that individuals with ADHD have different visual perception. It was reported that a group with ADHD presented spatial bias as part of impaired perceptual-attentional-action coordination.² In multiple visual-spatial tasks intended for right hemisphere processing assessment in children from 14 to 16 years old, right hemisphere dysfunction was reported, and adults with ADHD were also impaired on a letter cancellation task. Teenagers and adults with ADHD demonstrated a higher mean left-sided omission rate than healthy controls.^3,4 Visual perception problem in children with ADHD have been reported, however, little is known about the factors of visual perception in ADHD children.

Visual perception is viewed as an information-processing task involving the reception, organization, and assimilation of visual information in the central nervous system.⁵ If the information-processing centers of the brain do not run well, incomplete visual perception is the result. In several studies, hyper-reactivity to sensory stimuli in children with ADHD has been confirmed through parent-report and physiological testing,⁶ and they showed greater reactivity to sensory stimuli, with larger initial reactions and subsequent lack of habituation,⁷ which is a natural tendency to diminish the distraction created by incoming sensory information.⁸ In addition, children with ADHD often suffer visual hypersensitivity or photophobia, diminished ability to process auditory or tactile input, and poor balance performance, equilibrium, postural control or coordination.^{9,10,11,12,13} For these reasons, overactive and impulsive symptoms in sensory processing disorder can be easily confused with (and often co-occur with) ADHD, and children with ADHD often have comorbid sensory processing disorder.⁸ As a result, ADHD children who have sensory problems may also present different visual perception than children with ADHD who have typical sensory processing.

The main objective of the present study was to investigate the difference between children with ADHD with sensory processing disorder (SPD) and without SPD. It was hypothesized that children with ADHD and SPD would present with lower visual perception than children with ADHD without SPD, and sensory processing would be positively correlated with visual perception in ADHD.

It was hypothesized that children with ADHD and SPD would present with different visual perception than children with ADHD but without SPD, and that more accurate sensory processing would be correlated with more accurate visual perception in children with ADHD. The results of this study are consistent with these hypotheses.

The SPD group had lower mean scores on three quotients of K-DTVP-2 than the non SPD group, and the differences were statistically significant. We also observed that there was moderate relationship between general visual perception and sensory processing. There are several factors affecting visual perception, such as, attention, short- and long-term memory, individual's biases, feelings, attitudes, or life experiences.^19,20,21,22 While sensory processing is only one of the factors influencing visual perception, a moderate correlation could be considered quite meaningful.

The finding indicates that two groups clustered into SPD and non SPD by sensory processing ability have different visual perceptions. What causes the visual perception difference between two groups of children with ADHD? The results reveal that, the VMI score on K-DTVP-2 of SPD group was significantly different from the score of the non-SPD group. Also, general visual perception related to the specific SSP subscales 'movement sensitivity' and 'low energy/weak'. 'Movement sensitivity' is described as over-responsiveness to vestibular stimuli (e.g., 'dislikes activities where head is upside down', 'becomes anxious or distressed when feel leave the ground', and 'fears falling or heights'), and 'low energy/weak' is explained as a tendency to become tired and have weakness (e.g., 'tires easily, when standing or holding particular body position', 'poor endurance', 'seems to have weak muscles'). The results reflect that decreased sensory processing in these areas may be a hallmark of children with ADHD who have visual perception problems, and might indicate such when compared to typically-developing children. In a previous study, ADHD group displayed distinguishable 'low energy/weak' compared to typical children in SSP.²³ In addition, other studies suggested developmental coordination disorder (dyspraxia) often co-occurs with ADHD,⁸ and more than one third of children with ADHD have poor balance and coordination¹¹ associated with sensory inputs, sensory integration, and/or the inhibition of excessive movements.²⁴ Children with ADHD often have vestibular or proprioceptive problems which may be related to decreased visual perception.

Vestibular input and proprioception from eyes, neck and body, and visual information are integrated to form a "map" that is used to "navigate" the body successfully in space, they are unified into one composite sensory process in the brain stem. This sensory process helps us to direct our eyes when we look at things. If vestibular and proprioceptive sensations are not properly organized to keep the eyes moving smoothly, the child may suffer great discomfort when reading.²⁵ Therefore, children with ADHD and sensory processing disorder who suffer vestibular or proprioception process problems have deficits of focusing on an object to provide visual information, and of maintaining their posture to perform tabletop activities such as writing or reading. Also children whose major problem is in processing vestibular input may also score low in visual perception tests.²⁵

Visual perceptual problems can impact many areas of activities of daily living, especially school performance, for children. Sensory processing which affects several factors related to visual perception is essential performing visual-processing tasks such as matching shapes or objects, or tasks containing unfamiliar stimuli or subtle discriminations. Children may experience difficulty in recognizing similar letters or words, and may reverse letters and numbers in spelling. They may not be able to interpret charts, maps, graphs, symbols, and diagrams. In addition, activities that require fine motor and gross motor skills, such as handwriting and left/right discrimination can also be affected by visual perceptual problems.²⁶ However, visual perception is easily overlooked, while diagnosis and assessment are focused only on l more obvious dysfunctions in children with ADHD. Professionals may be overlooking a very important issue for children with ADHD. Without a visual perception assessment perceiving how they process visual information in their central nervous system, it is much more difficult to understand and help their daily activities such as performance in school. Sensory processing's effects on the visual perception of children with ADHD.

In summary, our findings confirmed the influence of sensory processing on visual perception in children with ADHD. The findings indicate that different visual perceptions do exist between children with ADHD with and without SPD. Also, sensory processing, especially vestibular and proprioceptive processing, is related to the visual perception among children with ADHD. This research can help to categorize potential difficulties of visual perception dysfunctions in children with ADHD. The limitations of this study are 1) control group of normal children was not involved, so it could not investigate the visual perception differences between ADHD children and normal children, however, through normative referencing, these children may be compared to their peers; 2) it had an insufficient number of subjects to examine parametric data analysis. However, the exclusion criteria of this study helped to strengthen its rigor; selecting children with ADHD without confounding factors such as medication adjustment allowed for a more accurate generalization of the results.

It is recommended that future works need to analyze sensory processing as a factor that affects the visual perception of children with ADHD, and to exam the effects of vestibular and proprioceptive senses related to movement and posture on visual perception. Using a greater number of participants in order to provide theoretical refinement of the impact of sensory processing on children with ADHD would be helpful. Also, investigating the correlations between sensory processing and visual perception for children with diverse diagnosis is required.