Empirical evidence suggests a phonological loop deficit associated with Down syndrome. A trisomy 21 may be associated with a narrowing of visual attention to fewer than four objects at a time too. In a study with computer tachistoscopy, the hypothesis was confirmed in all 194 persons with trisomy 21. The subitising limit of persons with trisomy 21 thus has a value that lies between the value of neurotypical persons and persons with simultanagnosia. A fitting term would therefore be ‘simultandysgnosia’. Consideration of this dysgnosia could prove to be a key to learning disabilities in mathematics in particular and learning disabilities in general. Diagnostic and learning methods should consider this limitation of the attention span not only in verbal but also in visual tasks.