Procedural memory enables us to carry out activities without giving them conscious thought 39, so a lack of procedural memory may increase our need to pay attention to our own hands, feet and objects 40; that is, the need to look, touch, smell and suck. However, autistic children can improve their procedural learning if they perform activities, the results of which are foreseeable and immediate: manipulating taps, spinning wheels of toy cars, or controlling lamps 4142. All these actions are called persistent preoccupation with parts of objects (DSM criterion) 438.

The less variability among the qualities of the objects result in an easier initial procedural learning 43, which also is achieved by environmental sameness 32; therefore, the marked distress over changes in trivial aspects of the environment (DSM criterion) 38, and the obsessive desire for sameness 2 may be justified.

A deficit of procedural learning complicates the development of automatic actions 2631, which apparently occurs during the first months of life in subjects with ASD 4445. Under these circumstances, they may react as typically-developing infants in a position to develop a calibration of movement directionality: looking at their limbs significantly longer as well as moving them more vigorously in order to increase procedural memory and self-knowledge 2646. Perhaps autistic children get the same result when increasing proprioceptive and tactile stimulation during swimming 47, using cylindrical hinged elbow splints 48, or controlling a self-immobilizing machine 49; therefore, rocking, swinging, spinning, flapping, finger flicking, tiptoe walking and jumping (DSM criterion) 4384041, might serve this purpose.

Using procedural learning, it is possible to develop postural control without perceiving our own sway, which plays an important role in the maintenance of posture 50; moreover, persons with ASD suffer underdevelopment of postural control 5152. Whereas persons developing normally have a 'frozen sway' so that the declarative memory can be free to focus on another task 53, some persons with ASD have a outstanding sway 4041, and they may be using their conscious thought to avoid falling.

A sequence of actions that does not vary is more suitable than several sequences for facilitating procedural learning 4243. This and the other strategies already described may explain the insistence on following routines in precise detail (DSM criterion) 438, but these strategies are not suitable for major development of procedural memory 324243; thus, they would decrease together with repetitive behaviors, although not all of them. Indeed, circumscribed interests are increased in high-functioning individuals with ASD 415455, which may be explained by the contribution of declarative memory to circumscribed interests. However, the increase in episodic memory is smaller than the increase in semantic memory in persons with ASD 56, since the contribution from procedural knowledge to retrieval of episodic memory may be faulty 57. This may explain why retrieval of sequences of actions to be executed is difficult for some high-functioning individuals with ASD 49, why they prefer reading a book (static elements) than watching TV (dynamic elements) or why they prefer either to playing video games (actions) 54, and why some autistic adults prefer playing an instrument or memorizing encyclopedic amounts of information although they have not yet learned to dress themselves nor tie their own shoes 958. Perhaps all this allows them to show their abilities instead of their disabilities caused by mnesic imbalance 304756. This may also explain the patterns of interest that are abnormal either in intensity or focus (DSM criterion) 438.

Procedural memory seems important for learning and categorizing of phonemes 5960, so a lack of procedural memory may lead to mutism, few phonemes or reduced vocabulary 61. However, many people with ASD would be able to sacrifice some acoustic features of the phonemes, such as amplitude variation and pitch level, in order to encompass all the phonetic categories of the native language, albeit with atypical features 62, producing a peculiar voice and prosodic deficits 4061; nonetheless, repetition may significantly improve procedural memory 26, which may convert some individuals with ASD into good imitators of prosody 63, who simply repeat what others said (echolaly) 4041, what they themselves said (palilaly) 64, or phrases that may seem irrelevant to the present situation 1. However, the speech thus acquired is not useful for communication because the meaning of words employed may be unknown to them 65 due to perceptual categorization that may require procedural learning 66. This may be the case with, for example, the categories of fruit and mental state expressed in eyes 1867, but even an autistic person with knowledge of categories would not be able to understand the meaning of sentences 3068 because the brain network associated with procedural learning is activated during sentence comprehension, likely, in order to 'reconstruct' in the brain the actions described by others 395069. Indeed, normally-developing children may use mental simulation to place themselves as the protagonist of a narration at the time they are selecting a deictic term, despite a contrary declarative knowledge 70; the same 'rebelliousness' is manifest when they say 'breaked' instead of 'broke' 71, so that procedural memory is used instead of declarative memory, resulting in inductive logic answers 72. All of this is in accordance to the declarative/procedural model, which assumes that morphology and syntax depend on procedural memory 3036. Faulty procedural learning may complicate the simultaneous application of the elements of verbal communication, for example the verbal intonation of children with ASD is better in repetition tasks (declarative memory) than in spontaneous speech (procedural memory) 73. This lack of simultaneousness also explains repetitive non-communicative speech; the difficulty in inferring appropriate words such as prepositions, adjectives, adverbs and deictic terms for the person (I, you, he, she, my, your, etc.), things (this, that, these, those), places (here, there, above, below, etc.), and times (now, tomorrow, yesterday); finally, sentences with grammar divorced from their context 417. All these are considered marked abnormalities in the production, form and content of speech (DSM criteria) 438.

Procedural learning is important to acquire phonemes and meanings 6066; this is why ironic speech may involve procedural memory 74. Indeed, the pragmatic difficulties in ASD seem to arise from inference deficits caused by faulty procedural memory 7275. Consequently, some persons with ASD initiate their 'conversations', with frequent irrelevant remarks, that are nonetheless very well practiced because they do not want to appear as fools; however, the failure of this strategy is obvious from the marked impairment in the ability to initiate or sustain a conversation with others (DSM criterion) 43847.

The absence of imaginative activity (DSM criterion) 38 might also occur because of the problems surrounding the learning of concepts and categories 66, and inductive reasoning 7576. In addition, the absence of real objects in simulation games may be a setback to faulty procedural learning, though that is an advantage for preserved procedural learning 324277.

Whereas the ability for inductive reasoning in children with ASD is poor, their deductive reasoning ability is good 7576, but their deductive reasoning ability may appear poor if the correct answers are inconsistent with the facts and it is difficult to disentangle what is more important 78. This means that fantasy can convert a deductive problem into an inductive problem, perhaps explaining the lack of interest among children with ASD in imaginative activities (DSM criterion) 38.

Some autistic persons can neither verbally communicate nor use gestural communication (DSM criterion) 438, even if they know the meaning of many words 3068. This may be because automatic instrumental gestures perhaps depend on procedural memory 152650, while lexicon depends on declarative memory 36. Indeed, automatic mimicry is impaired in autistic persons, whereas their voluntary mimicry is maintained 79, which explains why autistic people who have a good level of verbal comprehension have no trouble with instrumental gestures but do have trouble with expressive gestures 15, in other words, trouble with gestures to regulate social interaction (DSM criterion) due to instrumental gestures are easily translated to verbal language, while expressive gestures are not 188081. Additionally, autistic people who have a low level of verbal comprehension have trouble with imitation (DSM criterion) 3840. This would be related to the so-called 'mirror neurons' in children with ASD 82, since the familiar elements (procedural knowledge) of the observed novel movements might generate resonant activity within the mirror neuron system and thus facilitate procedural learning 83.

It has been proposed that gestural and verbal languages are essential for socialization and are not only instruments 84, so the lack of social or emotional reciprocity, the abnormal seeking of comfort, the lack of spontaneous seeking to share interests and the abnormal social play (DSM criteria) 438 may be viewed as result of impaired language. By contrast, these symptoms would be defined as impairments in dyadic orienting, joint attention and response to requesting 85.

The failure to develop peer relationships appropriate to developmental level (DSM criterion) 4 implies that autistic individuals are better at understanding physical systems than at understanding the minds of people. This is consistent with the empathizing/systemizing model, which proposes that systemizing works for deterministic phenomena with an exact explanation, whereas empathizing involves an imaginative leap in the absence of much data, whose causal explanation is at best a 'maybe' 86. It has been suggested that systemizing in individuals with ASD is good and their empathizing is poor 19; however, their systemizing may exhibit poor performance if systemizing problems are formulated as inductive problems 76, whereas their empathizing may improve significantly if the empathizing problems are formulated as deductive problems using explicit representations 87; consequently, some persons with ASD may be able to acquire an explicit theory of mind 5. Indeed, some persons with ASD use explicit mental representations to resolve systemizing problems, while normally developing individuals do not 4988.